Several years ago, federally funded researchers drafted a national health survey of 90,000 U.S. students in seventh grade and higher. The National Longitudinal Study on Adolescent Health, known as Add Health, became one of the most comprehensive studies ever of teenage health and behavior.

But a key behavior is missing from this monument of science. Working with the National Institutes of Health, project researchers included questions about oral sex, then removed them at the last minute. After vigorous debate, the investigators decided the project might not win congressional funding if questions about oral sex were included.

A similar reluctance to talk about any adolescent sexual behavior other than intercourse can be found among parents, doctors and teachers, according to those who treat or teach the young. "We don't tell young people how they can give and receive sex that gives pleasure and is not risky," says Debra Haffner, president of the Sexuality Information and Education Council of the United States. "We have nothing for young adolescents on how to negotiate sexual limits, and only a little bit for older teens."

Meanwhile, teenagers live with movies, TV shows and advertisements that all carry the same message: "Hi, how are ya? Let's do this, see ya," says Deborah Roffman, a sexuality educator in the Baltimore-Washington area. Without adult guidance, Roffman asks, "how are they supposed to know what to do?

Everyone agrees parents are the most effective guides. According to "Peer Potential," an analysis of more than 100 studies on peer influence by the National Campaign to Prevent Teen Pregnancy, "adolescents rely on peers for advice or assistance on sexual matters only when parental assistance is unavailable or inadequate."

But how can parents help their kids?

Ideally, assistance begins by teaching body parts to the very young child and remaining open to questions and discussion, says Haffner, author of a forthcoming book, "From Diapers to Dating" (Newmarket Press).

And as children pass through elementary school, Roffman notes, parents should inform themselves about the range of behaviors being discussed on the playground and use "teachable moments" to talk about those behaviors. "When boys start talking about girls and their breasts, adults should be right there," she says. "They need to preempt other influences." A raunchy TV episode, a sixth-grade dance where there was more bumping and grinding than dancing, even an affair in the Oval Office, provide such opportunities.

Talking to young people about the emotional and physical implications of sexual behaviors is extremely important, says Sarah Brown, director of the pregnancy prevention campaign. So is deciding in advance which behaviors the parent thinks are safe and appropriate, and communicating those values straightforwardly. One finding of the Add Health study, which related to intercourse but could be applied to other forms of sex, was that girls whose mothers clearly let them know they strongly disapproved of early intercourse were less likely to have sex than girls whose mothers were less clear.

Parents who are uncomfortable talking about sexual issues would do well to seek out a class for themselves and their child, says Melany Burrill, a sex education teacher for the United Methodist Church. Middle school is the time to do that, she says. "Anything after eighth grade is remedial."

Haffner agrees: "If you hand them a book in fifth grade and say only, `Read this,' don't expect they'll talk to you about oral sex in eighth grade." She quickly adds, however, that "it's never too late. You can always say, `I've done a crummy job. I'd like us to start talking now.' "

Conversations alone aren't enough, however. Parents must stay involved in their kids' lives to stay alert to what kids are thinking or doing. Haffner, the mother of a 13-year-old girl, says, "It amazes me how many parents don't know their kids have a boyfriend or girlfriend." She is also surprised by the opportunities for getting in trouble that adults unwittingly provide. "I have contemporaries who allow their daughter or son to have a friend over to study when there's no adult in the house."

Limiting a child's access to other kids who are sexually active can be helpful, the experts say; you can usually find out who they are by listening as your kids gossip with their friends and by asking other parents. Also watch out for young people who drink or take drugs; studies have linked such habits to early sexual promiscuity. However, having one or two risk-taking friends is not necessarily dangerous, according to "Peer Potential," if the rest of the young person's environment is healthy.

Professionals who offer this advice admit it's not fail-proof. Kids today "are not only one step ahead of us, they're one step ahead of our imagination," says Patricia Hersch, author of a book about Reston adolescents, "A Tribe Apart: A Journey Into the Heart of American Adolescence."

How true, says one mother in North Arlington, who learned after the fact that her eighth-grade daughter had been involved in several oral sex episodes. This mother thought she and her daughter enjoyed an open, trusting relationship and that she would know when her daughter was becoming intimate with a boy.

Medical professionals worry about the health implications for a whole generation of youth if parents and other adults don't do a better job of teaching young people the responsibilities of sex, including how to protect themselves. Already, one out of four young people will acquire a sexually transmitted disease before the age of 20, which puts them at risk of later infertility, cancer, heart or joint problems. Also, adolescents have the highest rates of gonorrhea and chlamydia of any age group.

And ominous signs hover on the horizon. Judith Wasserheit, director of the Division of Sexually Transmitted Disease Prevention of the Centers for Disease Control, says that for the first time in a decade CDC is seeing an increase in gonorrhea and chlamydia among men as young as 15, meaning some of them acquired the disease as young adolescents.

"The pressures on adolescents to initiate some form of sex are greater than they were in our generation," she says. "Parents and other adults have got to become significantly more knowledgeable about these issues so they can jump in and help adolescents protect themselves."

To learn more about the National Campaign or to order "Peer Potential," ($15), visit its Web site [http://www.teenpregnancy.org] or call 202-261-5655.