"You're kidding!" cried Jules Feiffer when a caller told him yesterday he had just won the Pulitzer Prize for his cartoons.

"This isn't some nasty joke, is it?" he said, and when the caller told him certainly not, this was for real and besides we were on deadline, he came around quickly. "I'll be right down to help you!" he laughed.

A minute later Universal Press Syndicate of Fairway, Kan., which handles the weekly cartoon for the 57-year-old satirist -- and which had nominated him -- phoned in with the news, and everyone began talking even faster.

"I must be slipping," he said. "I must be sliding to the right. I've been doing the cartoon out of The Village Voice since 1956 and this is only the second award I ever got. [The other was the George Polk Memorial Award in 1962.] But I'm delighted. I'm very excited."

He didn't mention that his plays have won him two Outer Circle Drama Critics awards, and "Little Murders" won a best-of-year award from the London critics and an Obie. Then there are his novels, from "Harry the Rat With Women" to "Ackroyd," and his screenplays ("Popeye," "Carnal Knowledge," etc.) and the other mordant satires that have been tumbling from his mind for 20 years.

"Actually, I was getting bored with the cartoons in the mid-'60s and wrote a play for a switch, but I found it reinvigorated me for the cartoons. Plus, the theater is so frustrating that I remembered I loved the instant gratification of newspaper work."

Today Feiffer lives in New York (where else?) with his second wife, journalist Jennifer Allen, who is working on a novel, and 15-month-old daughter Halley. He also has a 22-year-old daughter ("every 21 years I have a kid -- it's all in search of material") from his first marriage.

At the moment he is working on an essay about comic strips, a musical and a movie. His early cartoon book "Munro" (oh yes, that won an award too, an Oscar for the film version) is about to be reissued in yet another book, "Feiffer's Children." Munro, a 4-year-old who is drafted into the Army, has become part of the American landscape, at least for one generation of liberals.

"I think the liberal today is about like the liberal in the McCarthy era," Feiffer said. "Except without the terror of McCarthyism. Reagan's policies have so intimidated the liberals that they haven't been able to get themselves together. It's this so-called pragmatism of the conservatives: Pragmatism is what has got us into all this trouble."

The Age of Reagan gives him almost too much material, he says -- all too often the satire is built right into it -- and he finds it difficult to think up comments that haven't already been made by "all the other good people around these days."

But he still has his famous dancer, the earnest, leotarded young woman whose existential angst oozes from her very arms and legs. ("A Dance to Autumn: In this dance I celebrate the new Me . . . ") She has become a trademark for him.

"I've been drawing her since '56. The original was my first girlfriend, a modern dancer, the first to sleep over in my first apartment."

Meanwhile, Feiffer fans everywhere are doing a dance of their own. Paul Conrad, the Los Angeles Times political cartoonist who has won three Pulitzers, said yesterday it was a scandal that Feiffer hadn't won one before this.

"In '84 when I won my third," he said, "Feiffer was in town and we went out to celebrate. I told him then, the Pulitzer Prizes wouldn't be validated until he got one. Well, they finally are.