WORTH NOTING ON TV

* FRIDAY

Captain Planet and the Planeteers: an Earth Day marathon (The Cartoon Network, noon to 8 p.m.): Here's one way to make a potentially deadly subject appealing: Turn it over to Captain Planet and his teenage Planeteers. This is the second consecutive year the Cartoon Network has observed Earth Day with a marathon starring these pollution fighters. The Planeteers have special powers to fight environmental decline and are able to summon Captain Planet, a symbol of the Earth's strength.

Sixteen back-to-back episodes will be aired, with titles like ``Ozone Hole,'' ``Smog Hogs,'' ``Plunder Dam'' - well, you get the idea. The popular series, which debuted in 1990, has offered the voices of Meg Ryan, Whoopi Goldberg, Elizabeth Taylor, Sting, and other celebrities.

20/20 (ABC, 10-11 p.m.): The weekly news magazine includes an interview - taped in his home - with Louis Farrakhan, the black leader of the Nation of Islam, whose statements have caused an uproar among many groups and in the media. He may have the most feared mouth in American public life at the moment, but he hasn't met Barbara Walters on TV yet. I can hardly wait. She's tamed more fearsome figures than he. * SATURDAY

South African elections (C-Span, 1-3 p.m.): Elections take place April 26-28. Pre-election coverage includes profiles of candidates and parties, interviews with South African journalists, a look at social and economic issues, voter education programs, and other segments. (Coverage continues Sunday, same time period.) * SUNDAY

Bedtime With Barney (ABC, 7-8 p.m.): I suppose enough kids love Barney - on PBS and in the stuffed version - to justify noting the merchandising marvel's first appearance in a network special - in prime time, yet.

In this hourlong (!) saga, Barney and his friends travel to Imagination Island. There they encounter an eccentric toy inventor who discovers the joys of sharing. (Why I do I keep thinking of ``Bedtime for Bonzo''?)

Please check local listings for these programs.