The more hours teen-age children are left unsupervised after school, the greater the likelihood that they may be experimenting with sex.

This is a preliminary finding from a study of 400 middle school children between the ages of 12 and 15 in the Washington metropolitan area.

The children did not say they were experimenting with alcohol or drugs when they were asked what they do at home after school.

"Teen-agers these days don't get pregnant in motels and cars at 10 at night," said educational researcher Thomas Long. "Sex happens at home at 3 in the afternoon" while parents are at work.

Long, an associate professor at Catholic University of America, and his wife, Lynette, an associate professor at American University, plan to analyze and publish their latest study in a book soon.

The Longs, who have conducted other studies of children of working parents, estimate there are up to 10 million U.S. latchkey children under 14. These children are the offspring of two-career couples as well as single parents.

Children in single-parent families were most likely to experiment with sex. The Longs' study found that 40 percent of the children living in single-parent families said that at some time they have participated in heavy petting or intercourse at home while their parent was at work.

Latchkey children living with two working parents did not report having sex as often as those living with just one parent. Fifteen percent of these children said they at some time participated in heavy petting or intercourse while unsupervised.

But "the more regularly they were left unattended, the more likely they were to be engaging in sex," Thomas Long said.

The phenomenon of children taking care of themselves ishappening in upscale communities, rural areas, everywere," Lynette Long said.