The sexual activity of American teenage girls increased sharply throughout the 1980s, despite the risk of sexually transmitted diseases such as AIDS and an onslaught of public health messages urging them to abstain from sex, according to an analysis of a federally funded national survey released today.

While sexual activity increased among teens from all socio-economic backgrounds, the greatest increase was among girls who are white or from higher-income families, narrowing previous racial and income differences. In 1988, the median age for first intercourse for black females was age 16.6 and for whites 17.8 years.

"I think many people had expected a decrease in sexual activity because of all the media attention about AIDS and an administration where people were preaching about the importance of not having sex," said Jacqueline Darroch Forrest, vice president of research at the Alan Guttmacher Institute in New York, which released the study. "The results were a surprise."

The institute, a research organization associated with Planned Parenthood, analyzed figures from the National Survey of Family Growth, which conducted personal interviews with 8,450 women of child-bearing age. Other studies have examined the sexual habits of teenage boys and reached similar conclusions.

"The message of the 1980s for teenagers for preventing pregnancies and sexually transmitted diseases, particularly AIDS, was 'Just Say No,' " said James Trussell, professor of economics and public affairs at Princeton University. "These figures make it very clear that teens have not just said no. They said yes."

However, while more teens engaged in sex and had sex at earlier ages, there was also a dramatic increase in condom use, a development that public health officials hailed as encouraging.

"The good news," Trussell said, is that more couples are using condoms. "The bad news is that more teens are having sex and having sex at a younger age."

The proportion of girls aged 15 to 17 who were sexually active jumped from 23 percent in 1973 to 32 percent in 1982 and to 38 percent in 1988.

For females aged 15 to 19, the number of sexually active teens increased from 36 percent in 1973 to 47 percent in 1982 and to 53 percent in 1988.

"We had thought that increases in sexual activity might have reached a plateau in the early 1980s, but what we're seeing is a significant increase throughout the decade. That was unexpected," Forrest said.

A survey by the Urban Institute of teenage boys comparing sexual activity in 1979 and 1988 showed a similar pattern. "We were surprised how large the increase was," said Freya Sonenstein, a senior research associate at the institute. "We didn't believe the numbers at first."

In some socio-economic groups, such as Hispanics, teenage girls reported that condom use by their sexual partners tripled. For teenage girls of all groups, condom use doubled, a change in behavior that researchers attributed to the fear of AIDS and a growing acceptance on the part of society to discuss and promote condoms.

In 1982, 23 percent of all teenage girls said their partners had used condoms. In 1988, that figure rose to 47 percent.

"That's significant. It's hopeful. It's saying that people are hearing the messages about sexually transmitted diseases and the importance of condoms," Forrest said.

"That condom use is up is a good sign," Trussell said. "It is a fairly significant increase and very important as a public health finding. There was some fear that emphasizing condoms would mean that teen pregnancy might increase. But it is a really good thing that the increase in condoms was not offset by a decrease in the pill use. Oral contraception stayed about the same."

Many teenagers, however, still used no birth control. "The data are still saying that one-third of teens didn't use anything, not even the rhythm method," Forrest said.

"It is quite clear that there is substantially more condom use," Trussell said. "It is also quite clear that condoms are not being used all the time."

During the 1980s, births to teenagers increased and so did the incidence of sexually transmitted diseases. It is not known whether teenage births went up because the pregnancy rates went up, the number of abortion providers went down or because more teenagagers decided to give birth.