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.