William H. Masters, who with his psychologist partner, Virginia E. Johnson, monitored the tender act of lovemaking with science-lab instruments (and a bed) and became a best-selling author through frank books about sexual function and dysfunction, died Feb. 16 at a hospital in Tucson. He was 85 and had Parkinson's disease.

The pioneering work in sex research he conducted with Johnson, whom he later married, resulted in two unlikely hit books intended for scientific and professional audiences: "Human Sexual Response" in 1966 and "Human Sexual Inadequacy" in 1970.

Their research into orgasms and impotence, among other matters, prompted such a national discussion about sex that a Washington Post science writer in 1970 said their names had become "catchwords like Freud or Kinsey" and credited them in part with fostering "a new age of permissiveness."

Dr. Masters said he simply wanted to foster "a hopeful approach" for couples in need of marital guidance.

Dr. Masters, whose trade was obstetrics and gynecology at Washington University Medical School in St. Louis, started his long-term studies in the early 1950s, shortly after Alfred C. Kinsey had published two widely publicized studies about sexual behavior.

Where Kinsey relied on the memory and truthfulness of his patients, Dr. Masters wanted to see the act in a more immediate way. He captured sex with color movie cameras, floodlights and lab equipment, such as electrocardiograms, to gauge stimulation and its effects.

If successful, he said, he would help physicians, social workers and ministers develop a standard vocabulary with which to discuss sex in accurate detail and not make matters worse. Those in authority, he said at the time, had "no source of knowledge but their own experience -- which could have been good or bad."

Knowing that leaks might damage his reputation, Dr. Masters tried to keep his work top secret. But Washington psychiatrist Leslie H. Farber heard of the studies and wrote in Commentary magazine that Dr. Masters and Johnson were "scientizing" and dehumanizing sex.

Farber's arguments presaged the later critics and still others who disapproved on moral grounds. But Dr. Masters was steadfast in the scientific rationale behind the work, telling a reporter, "We are, of course, studying sexuality in the total context of human experience, but we had to start with the 'how' and 'what' of sex before we could get to the 'why.' "

When his full research was published in "Human Sexual Response," the reviews were largely favorable. Some noted the discovery that some women have a vaginal secretion that kills sperm in 10 seconds, a trait Dr. Masters called "the first natural contraceptive."

Dr. Masters found an early advocate in Mary S. Calderone, the founder of the Sex Information and Education Council of the United States. She said, "The Masters-Johnson research, I am convinced, can help society take a giant step toward the day when human sexuality . . . can be openly and freely taught -- to children, to young people who need such insight so desperately, and to their parents who need it even more."

William Howell Masters, a native of Cleveland, was a graduate of Hamilton College in New York and a 1943 graduate of the University of Rochester medical school. At Rochester, he worked closely with George Washington Corner, an anatomy professor who had helped encourage Kinsey.

Corner advised Dr. Masters in the ways of keeping beyond reproach in the nascent field. He told Dr. Masters to wait until he was 40 to begin his work to avoid arousing suspicion about his motives, secure his reputation in another field first and get the backing of a major institution.

Dr. Masters settled in St. Louis in the mid-1940s to join the Washington University faculty. He published pieces about the sex life of the aged before starting his larger sex project in 1954.

He dismissed his early experiments with prostitutes because he felt they did not provide an accurate account of how ordinary couples engaged in sex. As his reputation grew, he enlisted highly educated paid volunteers who included 382 women and 312 men and ranged in age from 18 to 89.

The research initially was funded by the National Institutes of Health, but after NIH declined to give further money in the early 1960s, private donors and organizations, including the Playboy Foundation, underwrote the costs.

Some of Dr. Masters's early findings were that a female orgasm is real and measurable (contradicting a view of some psychiatrists); that a woman's orgasms are more intense and last longer than a man's; and that the size of either partner's sexual organs has no bearing on sexual adequacy.

Over the years, their early work came to be scrutinized more closely and came under criticism for being based more on enthusiasm and belief than on fact. Dr. Masters insisted that his work be viewed as a first step in sex research.

"Let me use the example of the Wright brothers," Dr. Masters once said. "They got a plane off the ground. It would be wrong to fault them because it couldn't go to the moon."

The team's 1975 book, "The Pleasure Bond," was the first written for a mass audience. It was followed by "Masters and Johnson on Sex and Human Loving" (1986), among others.

In 1988, they and a co-author published "Crisis: Heterosexual Behavior in the Age of AIDS," a book that enraged the surgeon general by suggesting that AIDS was not being addressed adequately. Authorities disputed their research methods, but the authors insisted that the larger point was to curb promiscuity.

Dr. Masters moved to Tucson from St. Louis in the late 1990s.

His marriages to Elisabeth Ellis and Johnson ended in divorce.

Survivors include his third wife, Geraldine B. Masters; a daughter and a son from his first marriage; a brother; and two grandchildren.

William H. Masters teamed with psychologist Virginia E. Johnson.