Dr. William H. Masters, who with his co-researcher, Virginia E. Johnson, revolutionized the way sex is studied, taught and enjoyed in America, died Friday at a hospice in Tucson. He was 85 and had lived in retirement since 1994, first in St. Louis and then in Tucson.

He suffered complications from Parkinson's disease, said his wife, Geraldine Baker Oliver Masters.

Building on the pioneering work of Alfred Kinsey, Dr. Masters devoted more than half a century to observing, measuring, pondering and demystifying the mechanics of sexual intercourse and determining how to make the sexual experience better for couples who found its pleasures elusive if not unattainable.

The studies by Masters and Johnson, published in several books beginning with the 1966 best seller ''Human Sexual Response,'' played a major role in the establishment of the discipline called sexology, the treatment called sex therapy, an expanded role for sex education and a changed attitude toward sexual behavior, one that tried to be more informed, less fearful and more joyous.

Before Masters and Johnson, everyone talked endlessly about sex but few knew much about it. Misinformation was rampantly disseminated, and millions of men and women worried needlessly or were condemned to unrewarding sex lives. By studying at first the physiology of the sex act, they began to assemble data about sex that would be used to provide effective treatment of sexual problems and alleviate a great deal of misery, guilt and anxiety.

Dr. Masters's work essentially continued what was started by Dr. Kinsey, the most influential American sex researcher of the 20th century, who in the 1930's and 40's interviewed thousands of men and women about their sexual experiences and published two groundbreaking books about his findings, in 1948 and 1953. But while Dr. Kinsey was a scientist interested in accumulating data on human sexual experience, Dr. Masters was also a physician who was concerned with his patients' overcoming their sexual problems. As a result of the laboratory efforts of Masters and Johnson, material of clinical consequence was obtained that was used by therapists to treat sexual dysfunction and by educators to inform the public about how to have more rewarding sex lives.

When Dr. Masters began his work there were many in his profession who believed that sex was not a proper subject for scientific inquiry. This attitude, he said, supported the taboos and fears that were at the root of much of the sexual failure he sought to treat.

''One of the fun things of this work is to destroy old sacred cows,'' he once said.

The work of Masters and Johnson was sometimes received with condemnation, although not on the scale that Dr. Kinsey ran into. It was denounced because it emphasized sexual mechanics and not a loving commitment to a partner and because it did not condemn homosexual activity. In later years Masters and Johnson did tend to emphasize commitment more, but their primary focus remained physiological, not psychological, philosophical or moral.

They said that as far as they were concerned, any form of sexual expression between two consenting adults was acceptable and that any person, whatever the sexual orientation or problem, could come to them for help.

Trying to Avoid Controversy

The man who embarked on probably the most daring and explicit experiments ever conducted in the scientific study of sex was a church-going Episcopalian and a registered Republican who was generally cautious in his public statements and who sought to avoid controversy.

William Howell Masters was born Dec. 27, 1915, in Cleveland to Francis Wynne Masters and Estabrooks Taylor Masters, who were well off and who saw to it that their son was given an excellent education. He was sent to the Lawrenceville School in Lawrenceville, N.J., after which he attended Hamilton College in Clinton, N.Y. At Hamilton, he excelled both at science and athletics, and made the varsity squads in football, baseball and basketball. He was also a competitive debater. He graduated in 1938.

The following fall he enrolled at the University of Rochester Medical School and there came under the influence of Dr. George Washington Corner, who was conducting research into reproductive problems in animals and humans. By the time Dr. Masters earned his medical degree in the early 1940's, he had decided that he wanted to dedicate himself to studying the physiology of sex in humans. He reasoned that since it had always been one of the least explored areas of inquiry, it would be one of the most challenging.

Dr. Masters became interested in certain attitudes about the sexual experience, among them the 1758 assertion of Simon Andre, a Swiss researcher, that masturbation caused blindness; the research of Elizabeth Osgood Willard in the 19th century, who argued that the orgasm was more debilitating than a day's work in the fields and that sex for pleasure would ruin the body; and Sigmund Freud's belief in the 20th century that clitorally induced pleasure probably revealed unresolved psychological problems.

Dr. Corner, who had moved from Rochester to the Carnegie Institution in Washington, remained Dr. Masters's mentor. With Dr. Corner's guidance, Dr. Masters sought and received an internship in obstetrics and gynecology at the St. Louis Maternity Hospital and at Barnes Hospital, also in St. Louis. In addition, he studied pathology at the Washington University School of Medicine in St. Louis in 1944.

In 1947, he joined the faculty of the Washington University School of Medicine and became an authority on hormone-replacement therapy for aging women, a treatment that he advocated and that is widely used today. His early articles underscored the interest that was to become his life's work. Among them: ''Female Sex Hormone Replacement in the Aged Woman,'' ''Investigation of Sexual Regeneration in Elderly Women'' and ''Estrogen-Androgen Substitution Therapy in the Aged Female,'' all published in various journals in the late 1940's and early 50's.

In 1954 Dr. Masters began the research into human sexuality for which he would win international renown. It was then a controversial field of inquiry, but he thought it was essential to observe dysfunctional people trying to have sexual intercourse so that he would better know how to treat their problems. He received grants from the United States Institutes of Health and began his effort at Washington University.

Dr. Masters strongly believed that a woman should be involved in his research, and in 1956 he hired Ms. Johnson, a divorced mother of two, to help him interview people in sexual distress. She was not a college graduate but had studied psychology before becoming a singer with an orchestra directed by George Johnson, her first husband.

Dr. Masters divorced his first wife, Elisabeth Ellis, and in 1971 married Ms. Johnson, who now lives in St. Louis. She remained his wife and collaborator for 22 years, the union finally ending because, she said, she wanted to spend more time with family and friends and he remained deeply absorbed in his work. Dr. Masters married his third wife, Geraldine Baker Oliver, in 1993. She was the sister of his college roommate.

Dr. Masters is also survived by his children from his marriage to Elisabeth Ellis, Sarah Masters Paul of Weston, Conn., and William Howell Masters III of Manhattan; a brother, Dr. Francis Wynne Masters of Kansas City, Mo.; and two grandsons, Oliver Paul of Weston and Samuel Masters of Manhattan.

New Technology Aids Research

For a decade from the mid-1950's on, Dr. Masters and Ms. Johnson observed sexual activity involving 382 women and 312 men ranging in age from 18 to 89. Of these 694, there were 276 married couples who participated in the laboratory program. Their sexual activity was recorded on color film, along with their electrocardiographs and electroencephalographs.

New technology was a crucial element in the ability of Masters and Johnson to break new ground. Advances in the miniaturization of cameras and electronic devices meant that they could be used inside a plastic phallus. This permitted Masters and Johnson to record what took place inside the vagina during orgasm. In this way they were able to give definitive answers to some fundamental questions about sexual activity.

Masters and Johnson published their research in ''Human Sexual Response,'' put out by Little, Brown in 1966, their first book and one written for physicians. The language employed was deliberately arcane, because Masters and Johnson were anxious not to sensationalize their research or make it vulnerable to prurient interest. Thus the book refers to women undergoing orgasm as ''reacting units'' and to built-up sexual desire as ''tension increment.''

Despite such prose, ''Human Sexual Response,'' which contained specific clinical observations about coitus and masturbation and precise descriptions and measurements of the sex organs during arousal and orgasm, became a best seller, received high marks in both the academic and general press, and was translated into 10 languages.

Masters and Johnson always emphasized the therapeutic nature of their research. In 1959 they began counseling people with sexual problems as a natural outgrowth of their physiological studies. For this they developed a sex therapy team of a man and a woman and a specific methodology. Partners received two or three weeks of intensive therapy at their sex clinic in St. Louis, then follow-up consultation lasting as much as five years.

The results Masters and Johnson reported were impressive; they said that sexual difficulties were eliminated for 80 percent of the participants in the two-week basic program. Moreover, among those whose sex histories were examined five years later, only 7 percent reported a recurrence of the problems for which they initially sought treatment.

These results and more were published in a 1970 book called ''Human Sexual Inadequacy,'' which was also aimed at doctors and which also became a best seller. The book described successful treatments for premature ejaculation, impotence and the inability to achieve orgasm. An example of their findings (and their writing style) is the following passage: ''There is every reason to believe that maintained regularity of sexual expression coupled with adequate physical well-being and healthy mental orientation to the aging process will combine to provide a sexually stimulative climate within a marriage. This climate will, in turn, improve sexual tension and provide a capacity for sexual performance that frequently may extend to and beyond the 80-year level.''

Other Clinics Proliferate

Masters and Johnson are credited with popularizing sex therapy in America. Within two years after the publication of ''Human Sexual Inadequacy,'' sex therapy clinics began to proliferate across America. Some clinics were operated by therapists who were trained by Masters and Johnson, others offered different therapies.

Prof. Vern Bullough, in his book ''Science in the Bedroom,'' a 1994 history of sex research, says of Masters and Johnson: ''Before their entrance on the scene, the predominant treatment of sexual dysfunction, at least in the United States, was through psychoanalysis. What Masters and Johnson essentially did was challenge perhaps the final bastion of the control that psychiatry, and particularly psychoanalysis, had over the sex field.''

For a time, Dr. Masters hired prostitutes to act as sexual surrogates with partners who suffered from problems of impotency and frigidity. These surrogates caused controversy and in 1969 he abandoned their use because, he explained, he concluded that prostitutes were not suitable for the kind of experiences he was trying to understand in ordinary people and because he found that ordinary people themselves were willing to act as volunteer surrogates and be observed in a clinical setting.

By 1974 Dr. Masters was expressing fear that the field of sex therapy was being taken over by quacks and he was calling on the American Association of Sex Educators and Counselors to assert control through the same kind of regulatory mechanisms that existed in medicine.

Masters and Johnson had their critics, who called their reporting methods slipshod and not replicable. They also had prominent supporters, too, among them Dr. John Rock, a Roman Catholic advocate of birth control and a developer of an early contraceptive pill, and Dr. Mary Calderone, the executive director of the Sex Information and Educational Council of the United States.

Dr. Calderone said she hoped their work would make it possible to move toward the day when ''human sexuality can be openly and freely taught to children and young people who need such insight so desperately, and to their parents, who need it even more.''

By 1970 Masters and Johnson began to make their work more accessible to people other than physicians. In an article written for McCall's magazine in May 1970, they said they did not believe there was a ''sex act'' that a man performed on a woman. Rather, ''there are sexual relations between sexual partners,'' they wrote, adding that sexual expression, in whatever form it took, was a dimension of personality. They added that women did not have an ''equal right'' to an orgasm but instead an ''equal responsibility for it.'' Ms. Johnson wrote the article but it reflected the thinking of both researchers.

Professor Bullough, in ''Science in the Bedroom,'' said Dr. Masters in his research always paid particular attention to female responses, emphasizing that the female was not an inferior imitation of the male, which was a widely prevalent attitude at the time he began his work.

In 1975, Dr. Masters attracted attention when he addressed a convention of the American Psychiatric Association in San Francisco and asserted that any patient seduced by a psychiatrist should bring a rape charge instead of merely filing a civil suit for malpractice. The psychiatrists gave him an ovation.

That same year, the third Masters and Johnson book, ''The Pleasure Bond,'' was published. Unlike the first two, it was written for a general readership, and stressed aspects other than just the mechanics of sex. The authors said that sexual pleasure ''flows from the mutual fulfillment of wants and needs by a man and woman who are physically and emotionally committed to each other.'' This effort received only mixed reviews.

Masters and Johnson then wrote ''Homosexuality in Perspective,'' which was published in 1979. In it, they reported their experiences treating 84 homosexual men and women who were having difficulty with homosexual sex. They reported that they also treated 67 homosexual men and women who wanted to function as heterosexuals. The authors said that just over a third of them failed to sustain a longstanding reversal of their homosexuality.

Dr. Masters was appalled that some people thought he should not treat homosexuals. He asserted that homosexuals were as entitled to help as were heterosexuals, declared that homosexuals were neither emotionally sick nor genetically defective and that their sexual preference, like that of heterosexuals, was ''learned behavior.''

In 1988, controversy flared anew when they wrote ''Crisis: Heterosexual Behavior in the Age of AIDS.'' In this book, written with the collaboration of Dr. Robert Kolodny, who directed research for the project, they declared that the ''AIDS virus is now running rampant in the heterosexual community'' and would begin to attack the general population ''at a frightening pace.''

Among other things, they said it was theoretically possible to contract AIDS from such things as contaminated contact lenses, a salad made by an AIDS-infected restaurant worker or from instruments in a doctor's office. Dr. C. Everett Koop, then surgeon general of the United States, called the book ''irresponsible'' and critics said its science was thin.

In 1994 Dr. Masters decided to retire and closed the Masters and Johnson Institute, which he had founded with Ms. Johnson 40 years earlier. ''I've done a lot of therapy and at my age it's time to smell the roses a little bit,'' he said.

The Masters and Johnson therapy style has since fallen out of fashion and long-term therapy has become the prevalent method of treatment again. Therapists believed that sending a couple away for two weeks was unrealistic; they argued for treatment at home with real-life pressures. Even the team approach to therapy was abandoned.

Having two therapists proved too expensive and not essential, explained Dr. Shirley Zussman, a Manhattan sex therapist, who with her husband, Dr. Leon Zussman, were among the first therapists trained in the Masters and Johnson methods.

''People like to topple giants,'' she said. ''But make no mistake about it, they were giants. They are major contributors to the field of sexuality and to people's satisfaction and enjoyment of their sex lives.''

William H Masters