IT WAS THAT LAUGH, AN UNMISTAKABLE "aaawk, aaawk, aaawk," much like the mating call of an ostrich with bum sinuses. Week after week, millions of viewers heard Ron Palillo, as Arnold Horshack, laugh that laugh and croak, "Ooooh, Mr. Kattuh!" to actor Gabe Kaplan in the ABC sitcom about a Brooklyn teacher and his class of incorrigibles.

But since leaving Welcome Back, Kotter, in 1979, actor Palillo has had little to aaawk about. While fellow sweathog John Travolta gyrated to Saturday Night Fever fame, "nothing ever kicked in for me," says Palillo, now 38. "There were so many years of feeling that my life was a total waste, that it was over. I had sunk right into middle age."

Not to mention a crippling depression that left him in near seclusion at his Hollywood Hills home. Apart from occasional acting jobs—failed pilots like The Invisible Woman or the 1979 movie turkey Skatetown, USA—Palillo remained gripped by agoraphobic paralysis for the next five years. Finally, a reluctant visit to a psychiatrist to replace a sleeping-pill prescription revealed his condition and led to antidepressants that helped jolt him out of hibernation.

On a lark, he entered a pen-and-ink drawing in a celebrity auction in 1987 and, to his amazement, watched a bidding war ensue over the sketch. Then, two years ago, Palillo moved to Manhattan, a change that helped revive his career. He has since aaawked his way through the Mozart role in regional productions of Amadeus and will appear this September in Wind, a sailing film starring Matthew Modine and Jennifer Grey. He also has a part in an upcoming HBO comedy special starring Tom Arnold and has pursued his artwork, licensing his intricate Hobbit-like sketches for everything from shower curtains to T-shirts.

Three years ago, as a birthday present, Palillo treated himself to a nose job that, along with his Stairmaster workouts, he hopes will put the specter of his Kotter character behind him for good. After all, he says, "I finally have something more to say than 'I played Arnold Horshack.' "