Tonight at 6:30 turn the radio to WETA-FM, 90.9 on the dail, and let Jean Shepherd, the Prisoner of Radio, into your life.

Shep has done all kinds of things besides a nightly radio show. He's written four books, one of which, "In God We Trust, All Others Pay Cash," was a bestseller, and had 10 printings. He lectures at colleges, sells out oneman stands at Carnegie Hall, hosts a PBS show called "Jean Shepherd's America" which has gotten higher ratings than "Upstairs, Downstairs" - he's even had a play producted on PBS' prestigious "Visions" series.

"But very few people ever talk to me about that," Shepherd groused on the phone. "They say, "Hey, yeah, but let's talk about your real thing.' It's like if Laurence Olivier was known only as a great salesman for Polaroid, and critics kept writing, 'Polaroid salesman Laurence Olivier tried to do "Hamlet" last night.' If I won the Nobel Prize people would come up and talk about the all-night radio show."

There's a reason for all this radio talk. It's that Jean Shepherd does things over those airways that no one else can even get close to. He's a tale spinner, a mood weaver, a verbal artist, disciplined yet free-form, who can set you huddled over the set, hanging on his every nuance, his ever word. "Most people think of life as a some kind of never-ending struggle, a tragedy," he sayd. "To me life is a vast cosmic shaggy dog story, a giant, curiously unresolved joke with an infinitely long punchline."

To be heard only a week in Washington, Shepherd's current radio duties include a nightly, 45-minute show emanating from WOR In Manhattan. But the Shepherd era of legend, the period that captivated what seemed like every thinking being in the greater New York area, was the short period in 1959 when his shows would run to literally hours and hours of nonstop talk, spanning entire Saturday night from 9 p.m. to 1 a.m.

Maybe nonstop talk isn't quite accurate. For Shepherd could be counted rate. For Shepherd could be counted on to punctuate things with solos on the kazoo, the jew's harp, even his own head. There would be no telling, no predicting, what madness would come of the radio. He might, as he did one Saturday night, suddenly say, "Okay, I'm going to be silent for two minutes, and I want you to take your radio, put it by an open window, and turn the volume all the way up." The next thing you heard was Shepherd screaming, "I'VE HAD IT MILDRED, I CAN'T TAKE ANY MORE, I'M GOING TO MIAMI BEACH AND THE HELL WITH THE KIDS!"

Best of all were the stories he'd tell, weaving in and out of episodes, getting just to the good parts and then pulling back, saying, "No. I've only got three listeners after midnight and I have to save the ending for them." He'd talk of fishing for crappies in polluted lakes with his buddy Junior Brunner, or maybe tell of the time he was in the Army and spent a rainy afternoon going shopping with his girl. "She had a raincoat of her father's she wanted me to wear," he'd say, "and when she opened the closet" - here the voice would fade, so you'd hunch over some more, and finally pick up again, fairly vibrating with awe, "It was the raincoat of a two . . . star . . . general!"

Though Shepherd's people, his situations, seem terribly real, they are by and large created, and here again their creator thinks he's getting the short end of things.

"People say, 'He's not really creating, he's remembering. Norman Mailer's creating,'" Shepherd complains. "The less believable your characters, the more you will be hailed as a great creative writer. That dogged poor old Mark Twain to his death. People would say, 'How long has it been since you've seen Huck?' They believed he knew this guy. Twain really got bugged by it."

The reference to Twain is not accidental, because Shepherd considers himself, like Twain, "a humorist, not a comic. A comic tells predictable one-liners about show biz like 'Hey, did you hear the latest Agnew joke?' A humorist goes deeper, he deals with the process of life we all share."

Still, the fact remains that a lot of people who hear him simply have no idea exactly what it is Jean Shepherd is up to. "People will sit there splitting their pants laughing and then they say, 'What do you do"' sayd Shepherd, cantankerous as always. "I'm afraid," he adds sadly, "that I'm an original."