YOU MAY LEAVE the house in self-defense or plug into your own Walkman when your teen-agers play rock records. But if you have listened carefully to the lyrics -- or read them on the record jacket -- you know that most of the songs are not about the joys of raising a pet or the exhilaration felt on viewing Halley's Comet. Instead, sex and violence are prominent themes, with variations including incest, necrophilia and parent bashing. Things are different, though, in Mexico City, Lima and Bogota'. In these cities, and all over Latin America, the song at the top of the charts is a pitch -- believe it or not -- for chastity.

At first blush, this theme may sound as marketable as a rock-and-roll record promoting neat rooms, charity toward brothers and sisters and the wisdom of leaving the gas tank at least partially supplied. But cynics have been surprised. The idea for reaching teen-agers with this message comes from the Johns Hopkins School of Public Health, which received a grant from the U.S. Agency for International Development to devise innovative population programs for Latin America. Project director Patrick Coleman, who had served as a Peace Corps volunteer in El Salvador, saw the potential for reaching young people through music and put together a songwriter, a record company and two attractive teen-age singers to give it a try. The result: a hit record, on which teen-agers firmly declare that "it's not time to give ourselves everything" and confess "I say no even though my heart is burning." This is true -- honest. Sales have exceeded all expectations. A music video has been produced and a follow-up record is already a hit. There is more. Seven thousand youngsters wrote to a radio station to tell what the songs meant to them. Classroom discussions have been organized around the theme of the song.

Can this actually be happening? Will teen-agers buy the producer's message that "saying no is not only the responsible thing to do but something that is widely approved by peers"? And ultimately, will attitudes change enough to diminish even slightly the number of children -- now 2 million a year -- born to teen-agers in Latin America? The record results aren't in. And be advised: you're challenging a historic trend and biology here. But some population students are optimistic, and we hope they're right. Maybe the idea will travel. Can you imagine yourself buying more records for the kids and urging them to turn up that stereo?