Three-fourths of the teens polled said they knew how girls get pregnant, but they lacked practical information about making sexual choices, such as whether to have sex and about using contraception if they do.

The young people said their parents were among their most reliable sources for sexual information, but they said their folks don't give them all the information they need.

While three-fourths of teens said at least one of their parents has talked to them about sex, fewer than half said they had discussed birth control with them, and only 55 percent had discussed sexually transmitted diseases.

The report was based on a telephone survey by the Kaiser Family Foundation lerof 1,510 young people 12 to 18 years old.

It spotlights a dilemma for parents and educators. Many worry that discussing birth control and related topics in detail implies to children that they are expected to have sex.

``It's important that you not talk about sex in a vacuum,'' said Gracie Hsu, policy analyst with the conservative Family Research Council. ``Just more sex education in general is not necessarily good. It's what kind of sex education that's the difference.''

She said sex should be discussed in the context of marriage and relationships. In fact, many schools and churches across the country are doing just that with innovative programs that try to teach sexual values rather than simply provide technical information.

But others think kids need more basic facts to help them avoid pregnancy or sexual diseases.

Forty percent of teens surveyed said they get ``a lot'' of their information about sex from sex-education classes, but fewer than 10 percent of schools include all the factors needed for a comprehensive sexual education program, said Debra Haffner, president of the Sexuality Information and Education Council of the United States.

``What's most missing from sex education is sex,'' Ms. Haffner said.

Controversial topics such as pregnancy, birth control and masturbation were left out of most educational programs, Ms. Haffner said. ``But it's important to teach young people how to grapple with controversial subjects.''

"Parents are not automatically endorsing sexual intercourse by discussing sex with their children," said Robin Hatziyannis, the director of communications for Advocates for Youth, a Washington organization that works to prevent teen-age pregnancies and sexual diseases.

She said that parents should introduce sex to children as a natural part of growing up and begin talking about it as soon as their child broaches the subject.

While kids say they need to know more, there are signs that they are smarter about sex these days than they used to be.

Birth rates for teen-agers in the United States declined in 1994 for the third straight year, according to the National Center for Health Statistics.

The birth rate for teen-agers 15-19 years of age dropped to 58.9 per 1,000 in 1994 from 59.6 births per 1,000 population in 1993.

This trend may have been supported by a array of aggressive and innovative sex-education programs.

Communities that Care, a high school program started in 300 communities around the country with financial help from the Joseph P. Kennedy Jr. Foundation, teaches about sex in the context of five values: caring, respect, responsibility, trust and family. The program strongly encourages sexual abstinence.

Kathy Miller, who incorporates Communities that Care into her Family and Consumer Sciences classes at West Vigo High School inu West Lafayette, Ind., said a lot of teens choose sex simply because they think everyone else does. Once teens are able to discuss sex openly, their minds open.

``There are so many good teens out there who just need a platform,'' she said. ``Without discussion, they just assume everybody's having sex.''

Another National Adolescent Pregnancy Prevention Program, started by Dr. Michael Carera at the Children's Aid Foundation in Manhattan, gets the whole community involved in discussing sex with teens.

Families that participate in the program for a year are guaranteed a free education at Hunter College. Parents and children age 12-18 participate in separate ssgroups where they discuss sex and birth control in connection with academics, athletics and health.

Shavon Glover, who participated in the program at 15, after she already had a 1-year-old son, said it changed her attitude about sex.

``There were things I didn't know before I'd been in the program, that had I'd known, I'd not had a child at 14,'' said Ms. Glover, now 28.