Working with an initial group of 694 volunteers — 382 men and 312 women — Masters and Johnson hooked subjects up to instruments that recorded heart rate, brain activity and metabolism as they copulated or masturbated. Using a tiny camera placed in an artificial phallus, they were able to capture direct evidence, previously unseen, of what happens inside the vagina during female sexual arousal.

Among their findings were these:

■ Contrary to popular belief, there was absolutely no difference between a vaginal orgasm (the good kind, according to Freud) and a clitoral orgasm (the bad kind).

■ The length of a man’s penis has no bearing on his ability to satisfy his partner.

■ For elderly people, a group long considered sexually demure if not altogether chaste, vigorous sexual activity was not only possible but normal.

Ms. Johnson was often described in news articles as a psychologist, although in fact she never finished college. When Dr. Masters hired her, she was a divorced mother of two who had been a country singer, psychology student and writer. But as he often said, Ms. Johnson was precisely what he was looking for: an intelligent, mature woman who could help put his female subjects at ease.

Mary Virginia Eshelman was born in Springfield, Mo., on Feb. 11, 1925. An accomplished pianist and mezzo-soprano as a young woman, she performed country music under the name Virginia Gibson on a Springfield radio station, KWTO. She studied at Drury College in Springfield and the Kansas City Conservatory of Music and was later a business writer for The St. Louis Daily Record.

As Ms. Johnson said in interviews, she was raised to believe that a woman’s goal was marriage, and she took the injunction to heart. When she was very young, she married a Missouri politician; the marriage lasted two days. She later wed a lawyer many years her senior; that marriage also ended in divorce. In 1950, she married George Johnson, a bandleader, with whom she had two children. The couple were divorced in 1956.

Besides her son, Scott, Ms. Johnson’s survivors include a daughter, Lisa Young, and two grandchildren.