The number of reported cases of syphilis has dropped sharply during the past three years, and health officials say the decline has been most significant among homosexual men, indicating a major change in sexual behavior because of fear of AIDS.

"A falling syphilis rate is an extremely sensitive indicator of a change in gay sexual activity," said Ward Cates, director of the division of sexually transmitted diseases at the Centers for Disease Control. "Gay men account for more than half of the syphilis in the country."

CDC statisticians said that 12 percent fewer cases of syphilis were reported during the first half of 1985 than during the same period in 1984. Cates said the figure has fallen in almost every major metropolitan area.

Since 1982, the total number of syphilis cases has dropped 30 percent, compared with the previous five years, when the number of reported cases increased by 50 percent.

The falling syphilis rates may reflect a trend that began in 1983, when widespread fears about the transmission of herpes virus and the first national publicity surrounding acquired immune deficiency syndrome (AIDS) appeared to force a general reduction in levels of sexual activity.

Three elements suggest that sexual promiscuity has been vastly reduced over the past three years: the falling rates of syphilis, some figures which suggest a decline in one class of gonorrhea, and several recently conducted surveys of sexual behavior among homosexual men which show a marked decline in promiscuity.

"The evidence is as clear as it can be," said Casey Riley, director of Virginia's program on sexually transmitted disease, which reported a 25 percent drop in syphilis cases. "We went into VD clinics and asked people what was happening to their sex lives, and they told us that they are becoming monogamous because of the fear of AIDS."

One recent study of bisexual and homosexual men, conducted by the San Francisco AIDS Foundation, found that more than 80 percent of all homosexual men in the city have effectively reduced their risk of contracting sexually transmitted diseases by making sweeping changes in their sexual activity.

According to the study, more than half of those surveyed said they are now in a monogamous relationship, and the number of people who said they have more than one sex partner fell from 49 percent last year to 35 percent in the recent survey.

"The word is getting out there that the risks are not worth it," said Mark Behar, director of a national organization dedicated to improving education and resources about sexually transmitted diseases. "Education is the reason for these figures," he said.

Last year, the CDC reported that the number of cases of rectal gonorrhea, which most often affects homosexual men, had fallen dramatically in many major urban areas. In 1983, New York City reported a 59 percent drop in rectal gonorrhea cases -- its lowest figure in seven years.

The San Francisco AIDS Foundation now says that rectal gonorrhea in the city is all but eradicated, whereas five years ago it was a major health threat to the homosexual population.

"The rates of these diseases are falling off the charts," said Remy Lazarowicz, the western regional director for the American Social Health Association, an organization that closely monitors trends in sexually transmitted diseases. "You have to believe that behavior patterns have changed substantially throughout the country."

Although a connection has often been made between the decline of certain rates of VD and the wide fear of contracting AIDS, the number of cases of other sexually transmitted diseases -- those less often associated with homosexual men -- has risen dramatically over the past several years.

AIDS is a fatal disease that disables the body's immune system, but both syphilis and gonorrhea are sexually transmitted bacterial diseases that are easily treated with antibiotics. Public health officials have said that the biggest deterrent to fighting venereal diseases is a lack of public funds for treatment and education.

According to the American Social Health Association, more than 40 million new cases of venereal diseases have occurred in the United States since 1980.

The association estimates that one of every four adults in the country will eventually be affected by some form of VD.