For almost as long as memory runneth backward, Fred Fiske has been a big voice as both a disc jockey and a talk-show host behind the microphone of Washington radio stations. This native New Yorker began locally with WOL in 1947, shifted to WWDC in 1950 and, after that station changed formats in 1977, moved to WAMU-FM, a public station sponsored by the American University.

The main point of the story here is that Chevy Chase resident Fiske returned to the air last night after a five-week hiatus forced by what he describes as a "mild heart attack." A group of listeners, headed by that omnipresent actionist Carlos Van Leer, rallied outside the WAMU studio last night to greet Fiske's return.

Van Leer outdid even himself a bit, standing earlier yesterday at one of the traffic portals to Washington holding a sign proclaiming that "Fred's Back . . . 88.5 FM."

On June 30, returning home from a crab feast at the Southeast Washington home of Mayor Marion Barry, Fiske said he felt chest pains while on the Southwest Freeway. He decided that if the pains persisted when he got into the Palisades area, he'd stop at Sibley Hospital and have them checked out. They did. He stopped. And he was admitted with the heart-attack diagnosis.

Fiske said he will "go about living just as I did before, with just some minor modifications in diet and some medication," and doing his interview and talk show.