WETA's "AIDS: Changing the Rules" is, in part, light treatment of a heavy subject, but it manages to achieve just the right tone for its purpose -- to tell young, heterosexual and sexually active people all about AIDS and, in so doing, to tell them some things about sex they might not already know.

You have to know about AIDS to know why you don't want to get it. You have to know about sex to know how you can or cannot get or give AIDS, and you have to know about both sex and AIDS in order to know the best ways not to get AIDS when you have sex.

"Some people," says top fashion model Beverly Johnson, one of the show's narrators, "are panicking about what they've done in the past ... {but} heterosexuals haven't really been at risk until now ... Some people are deciding never to have sex again -- with anyone, ever. Abstinence is the best protection of all, but it's not very realistic for most of us. It's not my solution," she says. "If it's not yours there are some things you need to know."

The hour-long "Changing the Rules," with Johnson, popular singer and actor Rube'n Blades and first son Ron Reagan, explodes some myths ("only gays get AIDS") and confronts some really tough questions, like protection during oral sex, but presents its principal thesis with credibility, intelligence and a lot of humor. The major discussion concerns the use of the condom, and it's full of double-entendres. The program's young narrators make no attempt to avoid them. In fact, they meet them head on, embrace them, subsume them into the ultimate purpose of the program -- with enormous effect. It defuses the embarrassment, somehow, without lessening the horror of AIDS.

Blades, who has recently added a Harvard master's degree in international law to his credits as singer, composer, writer and actor, has the toughest job. He demonstrates the use of the condom. This is essential, as he notes, because "condoms are the best protection we have against giving and getting AIDS -- but only if you use them right."

He produces the condom, shows how to apply the spermicide Nonoxynol-9 (which kills the AIDS virus) and then takes a deep breath. "This is a banana," he says. "Sure wish there were an easier way to show you this." But there really isn't. So he makes do.

The second half hour of "Changing the Rules" is a discussion of some of the topics alluded to earlier, anticipated as a starting point for discussions in classrooms or with parents. WETA, Channel 26, is broadcasting the program tonight, tomorrow and Thursday at 11 p.m. and at 2 p.m. Friday, a school holiday. Response to its first showing last Friday has been moderate, a WETA spokesman said, noting that about 30 videocassettes of the program have been ordered since the broadcast. The program, aimed especially for use in schools and by young people's organizations, will be offered for sale nationally. The first half hour, produced by AIDSfilms Inc., can be ordered separately and with Spanish subtitles.