Laurance S. Rockefeller, the middle brother of the five prominent and philanthropic grandsons of John D. Rockefeller, who concentrated his own particular generosity on conservation, recreation, ecological concerns and medical research, particularly the treatment of cancer, died yesterday at his home in Manhattan. He was 94.

The cause of death was pulmonary fibrosis, according to a family spokesman, Fraser P. Seitel. Mr. Seitel said that as recently as Wednesday, Mr. Rockefeller, whose health had declined in the last several months, had gone to work at his offices at Rockefeller Center, where he joked with the staff that he was putting in only half a day.

Mr. Rockefeller's career began on Wall Street almost 70 years ago, and he became a trailblazer of modern venture capitalism, compounding his inherited wealth many times over. In the decades since he first took his seat on the New York Stock Exchange, he often used his native instinct for identifying the next big thing. He was not content with simply making more money but wanted to make the money produce something of lasting value.

In the late 1930's, he provided much of the capital to help Capt. Eddie Rickenbacker, the World War I fighter pilot, start Eastern Airlines and was for many years the airline's largest stockholder. He invested -- personally or through a family partnership he established -- in hundreds of start-ups, initially in aviation and electronics and later branching out to computers (Intel and Apple) and biotechnology. His gifts of land, from Wyoming to the Virgin Islands, amounted to hundreds of thousands of acres, including a prominent New York vista that contained a vast unspoiled tract along the west bank of the lower Hudson River: Tallman Mountain State Park.

Less gregarious than his older brother Nelson, the four-term governor of New York and the vice president under Gerald R. Ford, Laurance Spelman Rockefeller was also more reserved and private than his flamboyant younger brother Winthrop, who was governor of Arkansas. A philosophy major at Princeton, he long wrestled with the question of how he might most efficiently and satisfyingly use the great wealth into which he was born.

In the process, he shaped a life that was also distinct from that of his oldest brother, John D. 3rd, who devoted himself to maintaining the charitable efforts established by his father and grandfather, and of his youngest brother, the scholarly David, who headed the Chase bank and was deeply involved in issues of international finance and politics.

David Rockefeller, who is the last surviving grandchild of John D. Rockfeller, said yesterday of Laurance, ''His inspiration and wisdom will be missed by every generation of the Rockefeller family and most especially by me.''

In his 1997 biography ''Laurance S. Rockefeller, Catalyst for Conservation,'' Robin Winks said that more than 200 books had been published about the Rockefeller family. He wrote that ''one may read all these books, some friendly, some angry, to discover that there is one Rockefeller who is barely present in most of them. This is Laurance, who moves in and out for a page or so and is then gone, neglected by an author who sometimes is in search of scandal or the stuff of headlines, or whose interests are in politics, high finance, or international affairs.

''As a result, most of these books have neglected the Rockefeller who, in the tradition of grandfather and father, arguably has moved and shaken to the most long-range purpose -- the preservation of the nation's natural heritage, of great historic landscapes.''

Using significant amounts of his money, as well as his connections, prestige and negotiating skills, Mr. Rockefeller was instrumental in establishing and enlarging national parks in Wyoming, California, the Virgin Islands, Vermont, Maine and Hawaii. In his home state, New York, he expended further cash and influence to help establish parklands and urban open spaces.

In addition, there were projects he thwarted, including the encroachment of high-rise developments along the Palisades north of the George Washington Bridge. There, as an active member of the Palisades Interstate Park Commission, he helped create a chain of parks that blocked the advance of sprawl and maintained the majestic view that he first saw as a child looking out from Kykuit, the Rockefeller country home in Pocantico Hills, in Westchester County.

Mr. Rockefeller served on dozens of federal, state and local commissions and advised every president since Eisenhower on issues involving recreation, wilderness preservation and ecology. He founded the American Conservation Association and supported scores of other environmental groups.

In 1991, awarded the Congressional Gold Medal and extolled by President George H.W. Bush as a ''hidden national treasure.'' Mr. Rockefeller said that that nothing was more important to him than ''the creation of a conservation ethic in America,'' and then he sternly admonished the politicians in the room to do more to protect the country's land, water and air.

His interest in nature and preservation extended to commercial ventures. As the founder of RockResorts, he helped design and build hotels in imposing natural settings from the Caribbean to Jackson Hole, Wyo., and Hawaii, always emphasizing close access to nature. These environmentally focused hotels, built between 1955 and 1969, contributed to the growth of what has since become known as eco-tourism.

An unabashed apostle for the restorative powers of nature in an age of anxiety and complexity, Mr. Rockefeller revealed an unusual mix of ingenuousness and perhaps chutzpah when he wrote an article for Reader's Digest in 1976 called, ''The Case for a Simpler Life.''

The author, who owned paintings by Bonnard and Gainsborough, many large houses, a plane, a Bentley among other cars and a 65-foot speedboat, cited his family's view that waste was always sinful. ''The concept that we have boundless resources of materials, manpower and spirit and therefore can waste, clearly no longer is true,'' he wrote. ''Individually, people are finding that a simpler lifestyle provides greater satisfaction than relentless pursuit of materialism.'' He wrote that in his case, ''chopping and splitting firewood'' brought him spiritual and mental relaxation along with exercise.

A Love of Nature

Mr. Rockefeller's commitment to wilderness, recreation and environmental conservation had many roots. Since childhood, he liked to ride horses through unspoiled terrain. He was a passionate photographer who sought out new landscapes.

As a young man, he traveled to national parks in the West. In Yellowstone, he met and formed a lifetime friendship with Horace M. Albright, the park superintendent who would become a driving force in the conservation movement and was one of Mr. Rockefeller's two acknowledged mentors.

The second of these was Fairfield Osborn, a man who retired young from Wall Street to lead the New York Zoological Society and who sounded the alarm of ecological threat in a book called ''Our Plundered Planet.'' Mr. Rockefeller referred to him affectionately as ''a rogue elephant'' railing against despoilers of the environment. In 1972 he wrote an article about Mr. Osborn for Reader's Digest that ran under the rubric of the magazine's regular series, ''My Most Unforgettable Character.''

Even before Mr. Rockefeller reached adulthood, his family had included parks among their many philanthropic projects. John D. Rockefeller Jr. had donated some of his land on Mount Desert Island in Maine to help create Acadia, the first national park east of the Mississippi River, in 1919, and 10 years later he donated much larger tracts in Wyoming to help establish Grand Teton National Park. But even these projects were overshadowed by many of the family's better-known endeavors in education, race relations, religion and science.

As a young man, Mr. Rockefeller became steadily aware that the philanthropy of his generation would require greater specialization than the eclectic approaches at of his immediate ancestors. ''My grandfather gave like a forward pass,'' he said in an interview with The New York Times in 1966. ''He threw the ball out, and it was up to the others to make a go of it.'' Then referring to himself and his brothers, he added, ''We feel you must give with the heart as well as the head. There must be a deep personal commitment along with an intellectual understanding of the project.''

Wealth and Responsibility

Laurance Spelman Rockefeller was born on May 26, 1910, at a time when the health of his grandmother Laura Celestia Spelman Rockefeller, known as Cettie, was beginning to fail. It was to honor her that the boy's name was spelled unusually, with two a's. As Mr. Rockefeller grew older, he came to resemble his grandfather more closely than any other family member, having the same pursed and seemingly dour expression that John D. Rockefeller often showed in photographs.

According to family accounts, he also was the one who most closely showed his grandfather's flair for profitable deals. The three generations lived together in New York City and at Kykuit.

It was by all accounts an unusual childhood.

As a boy, Mr. Rockefeller was a tinkerer, or as he later called himself, a gadgeteer. He built and rode a wooden car that he attached to a small motor. He was closest to Nelson, who was two years older, and the two called each other by the names they preferred, Nelson answering to ''Dick'' and Laurance to ''Bill.'' At one point, the two started raising rabbits, which they sold to scientific laboratories. Along with their brothers, they were paid for chores like cleaning shoes. The compensation for swatting flies was 10 cents for 100 flies.

Their school was an experimental and progressive academy called the Lincoln School in Morningside Heights. It was founded by John Dewey and devoted to a methodology of learning by doing. Hobbies like Mr. Rockefeller's interest in photography were encouraged, as were travel and exploration. Disciplines like spelling were less stressed. Laurance and Nelson would often start off for school on roller skates with a chauffeur following behind, ready to drive them on the last leg of their journey.

Mr. Rockefeller, his brothers, and their sister Abby, the oldest child of John D. Rockefeller Jr., grew up in what was almost certainly the richest family in America, yet frugality was a part of their upbringing. Their father regularly reviewed the accounting books where his sons kept track of their expenditures, just as his own father had done. John D. Jr. offered his sons gifts of $2,500 if they would not smoke before they turned 21. Only David and Nelson could claim the prize.

In building Kykuit, the family had spent more money on shaping the grounds than on the house, which had no ballroom or bar, since the elder Rockefellers disparaged dancing and alcohol. Though Laurance was raised amid advantages and luxury, he also was exposed to values like asceticism and economy, which emanated largely from the family's Baptist beliefs.

After entering Princeton in 1929, Mr. Rockefeller began challenging some of these religious convictions. He said he felt as if he had ''always lived with a Bible in one hand,'' and he became a philosophy major in order to determine whether faith or reason offered the best ways of identifying and obtaining what was good.

In the throes of his quest, while taking every philosophy course Princeton offered, he wrote to his father, ''I am of the opinion that the appreciation and the desire for what is good takes more study and insight than does the understanding and test for the best music and art.''

A Knack for Business

After Princeton, Mr. Rockefeller went to Harvard Law School, but after two years he realized he did not want to be a lawyer. In 1934, he married Mary French, the sister of Nelson's roommate at Dartmouth and the granddaughter of Frederick Billings, a president of the Northern Pacific Railroad.

The couple had three daughters and a son, all of whom survive. They are Laura R. Chasin of Cambridge, Mass.; Marion R. Weber of Stinson Beach, Calif.; Dr. Lucy R. Waletzky of Pleasantville, N.Y.; and Larry Rockefeller of New York City. Mr. Rockefeller is also survived by his brother David, eight grandchildren and 12 great-grandchildren. His wife died in 1997.

Around the time of his marriage, Mr. Rockefeller turned to business. When his grandfather died in 1937, he inherited his grandfather's seat on the New York Stock Exchange.

But instead of accumulating blue-chip holdings, he began investing in embryonic undertakings. One of his first such efforts happened when he and his bride, while furnishing their first home, were shown photographs of curved wooden furniture designed by Alvar Aalto, a Finnish architect. The couple ordered a number of pieces for themselves, but Mr. Rockefeller ordered many more, believing that Aalto's work would attract buyers seeking good modern design that was also affordable. Ignoring furniture experts who warned him against it, he opened a store on 53rd Street to sell the Finnish furniture.

A second venture was to have much greater consequences. Mr. Rockefeller had become friendly with Captain Rickenbacker, who had triumphed in many dogfights over Europe. Mr. Rockefeller had himself learned to fly, and he found Captain Rickenbacker's visionary accounts of an approaching boom in commercial air travel to be persuasive. Within a decade after Mr. Rockefeller's considerable investment, Eastern Airlines had become the most profitable airline to emerge after World War II.

During the war, Mr. Rockefeller served as a Navy procurement officer. But afterward, he resumed searching for start-up opportunities. In 1959, an article in The Wall Street Journal noted that Mr. Rockefeller has ''no thought of abandoning his 20-year-old policy of using his wealth as 'venture capital' on untried new enterprises,'' the quotation marks around the words venture capital suggesting that the term was still novel.

The Journal article reported that since the end of the war, Mr. Rockefeller had invested about $8,750,000 in some two dozen companies and that by the end of 1958 these investments had grown to be worth more than $28 million.

As Mr. Rockefeller kept identifying ground-floor investment opportunities, his brothers and his sister would from time to time join in particular investments. In 1969, he formalized these activities by establishing Venrock Associates, an investment company that was a limited partnership financed by members of the Rockefeller family and a number of the institutions with which the family had longstanding philanthropic ties, among them the Museum of Modern Art, Rockefeller University and Memorial Sloan-Kettering Cancer Center.

By 1996, Venrock had financed 221 initiatives in the early stages of their development, amassing an impressive performance record. After Mr. Rockefeller's early concentration on aviation, he branched out into electronics, computers and biotechnology, providing capital early on to companies like Intel and Apple long before they became widely known and highly valued.

According to Mr. Winks, the biographer, Mr. Rockefeller, through his investments, ''increased his personal fortune many times over.''

Protecting the Environment

Though his business pursuits were hardly devoid of passion, it was his desire to simultaneously protect the wonders of nature while providing greater access to them that aroused his greatest enthusiasm. In 1939, he was first named a member of the Palisades Interstate Park Commission, and in 1942 he made his first gift of land, providing the tract that is now Tallman Mountain State Park. In 1949, Mr. Rockefeller signed over more than 30,000 acres that his father had assembled at Jackson Hole to the federal government. Within five years, he had built three lodges offering visitors comfortable accommodations abutting Grand Teton National Park. In 1960 he became vice chairman of the New York State Council of Parks under Robert Moses.

In 1952, while cruising in the Caribbean with his wife, he put in at Caneel Bay, a cove fringed with frangipani, bougainvillea and monkey-no-climb trees on St. John, the smallest and most unspoiled of the United States Virgin Islands. Over the next four years he assembled an area of 50,000 acres that he turned over to the National Park Service as Virgin Islands National Park.

From 1958 to 1962, he served as chairman of the Outdoor Recreation Review Commission that President Eisenhower appointed to study the country's recreation needs. Many of the reports recommendations for the designation of new parks and the expansion of old ones were enacted, particularly during the Johnson administration.

As the middle son, Mr. Rockefeller had long experience in promoting compromise. David Rockefeller has described in his memoirs how Laurance repeatedly sought to ease tensions between David and Nelson as they squabbled over who should be chairman of Rockefeller Brothers and what should be done with the family estate in Pocantico Hills. In public policy disputes, Mr. Rockefeller's also often took up the role of mediator.

In the mid-1960's, President Lyndon B. Johnson asked Mr. Rockefeller to push forward a long-stalled plan for a Redwood National Park in California that had first been suggested in 1917. Naturalists led by the Sierra Club were insisting that the envisioned park be mapped to include the largest possible area of giant trees, while the lumber companies and state politicians countered with proposal of much smaller acreage.

Mr. Rockefeller engineered a compromise with the State of California that failed to please many. Instead of the 77,000 acres that the Sierra Club had sought, the national park that was established in 1968 contained 58,000 acres. It was the costliest park in the system's history, with $1.5 billion spent on land alone and hundreds of millions more going to displaced lumbermen. Yet in time, the recognition grew that if not for Mr. Rockefeller's compromise there would have been no park at all.

Another controversy concerned Mr. Rockefeller's effort to have New York's huge Adirondack Park, the largest state park in the country, be put under federal control as a national park. Starting in 1961, Mr. Rockefeller commissioned a series of studies that suggested that turning the park, which is the size of Vermont, over to the National Park Service would be the most effective way to reverse commercial incursions by timber companies and others, since federal regulations on land use were more stringent than those of the state.

The opposition of local residents and politicians ultimately swamped the idea, but Mr. Rockefeller's continued emphasis on the park's importance has been credited with reversing years of neglect, deterioration and exploitation as the state government asserted greater control.

It was at Caneel Bay that Mr. Rockefeller opened the first of his RockResorts in 1956, seeking to test his belief that there could be an alliance between commerce and conservation. In designing the place, he determined that his guests were to swim, snorkel, sleep, eat well but not luxuriously and take nature walks, There would be no telephones, no air conditioning and no tipping.

Mr. Rockefeller relaxed some of these restrictions as he built and operated other resorts, among them the Dorado Beach Hotel in Puerto Rico, Little Dix Bay in the British Virgin Islands and the Mauna Kea Beach Hotel in Hawaii. In 1985 he divested himself of the RockResorts, adhering to the investment principles he outlined when he set up Venrock: ''Get in on the ground floor, stay with the enterprise until it reaches maturity, then get out and move to something else.''

Laurance S Rockefeller