David Letterman is worried sick. David Letterman is scared silly. But these are natural states for him. This is part of what makes him Dave.

"I'm terribly nervous about this. I'm just riddled with anxiety," Letterman says when asked about "Late Night With David Letterman Sixth Anniversary Special," the 90-minute prime-time celebration airing tonight at 9:30 on Channel 4. Though ratings for his regular postmidnight "Late Night" show are better than ever, Letterman notes fretfully that NBC demands much larger crowds to show up in prime time.

"When we do really good business, it's just like spit on the windshield compared to what the prime-time audience is," Letterman says. "And especially Thursday night on NBC. I think we could really end up making fools of ourselves.

"I really believe that if this thing tanks, I'll never go near prime time again."

But Dave, Dave, Dave. Dave? Dave! The ratings will probably be fine. You're sure to win your time period. "On Thursday night, winning the time period is nothing," counters Dave. "I really feel this is a pivotal point in the career of this show.

"It's the GE people, it's the NBC people. It's like, 'Well, what has he done lately?' It means something in the back of the minds of the programming people if they say, 'Well, we gave Letterman every possible advantage and the show did okay.' They ain't looking for a show in that time period to do okay. They're looking to march through Poland."

All this worrying from NBC's fair-haired boy. From NBC's fair-haired Peck's Bad Boy. Although at 40, Dave is turning more into Peck's Bad Brother-in-Law. And his hair isn't that fair either, come to think of it. But he and his cohorts have made "Late Night" the funniest and most trend-setting comedy show in all of television, and that's a lot of "all."

Indeed, if one steps back and looks at the '80s, which is admittedly not a pleasant thought (the stepping back, yes, the looking at, no) you'd have to say that most of the really funny things that have happened in this decade that have not involved politicians or evangelists have happened on the David Letterman show.

Tonight's special, produced for a relatively lavish $1.5 million, will include new sequences staged at Radio City Music Hall, "stupid pet tricks" brought back for encores, and taped highlights from past shows. Anyone who missed it the first time will get to see Dave menaced and attacked by one of Rena Smaha's trained monkeys, a feisty ape in a print dress that took a strange and immediate dislike to him.

This was a case where Dave's paranoia proved entirely justified.

For the finale, Letterman will reprise an old trick but with grandiose new dimensions. He will be bounced and catapulted onto a 30-foot-tall Velcro wall and hang there like a fly. NBC publicity has described this as a heart-stopping thrill. "It's far from heart stopping," Letterman says, "unless you've got one of those faulty Jarvik deals."

And then of course there will be the Radio City Music Hall Rockettes. "When they come out, these 36 very leggy women all lined up in their little goose outfit or whatever it is, and they start the kick line, as corny as it sounds, it's like watching fireworks," says Letterman. "You just go, oh wow."

He sounds so up!

"And then the show goes downhill from there."

He sounds so down!

In fact, Letterman hasn't seen the finished show yet. He was still refusing to watch the final edit yesterday afternoon. Producer Robert Morton says he tried to show Dave one of the musical numbers and Dave just threw up his hands and ran out of the room, or something along those lines.

He is worried about the ratings because NBC's Thursday night lineup is week-in and week-out the highest rated night in all of television. It starts out with "The Cosby Show," which gets a 47 share (meaning almost half of all the sets turned on are tuned to it), continues with what Letterman calls "that Cosby annex show, whatever the hell it is" (the sitcom "Different World") and stays big with "Cheers."

"So for me this is the biggest potential crossroads in the history of the show," Letterman says.

On the air, night after night, Letterman often appears anxious, peevish, riddled with self-doubt, pessimistic to a biblical extent, given to anguished grimaces and beset with hallucinatory delusions that "it's hot in here." Is this the real Letterman? In real life, he seems much the same, an extremely disarming neurotic wreck.

"I think when you do a show like this night after night, you're going to have to be a bigger version of the person you actually are," Letterman says. "And you're going to react more strongly to things that you would not react so strongly to in your own life. I like to think it's kind of an honest representation of the person I am."

He does love to worry. He studies the "Late Night" ratings microscopically and no matter how high they are, frets that they will fall. Of late he and his staff have been disconcerted by changes made in Johnny Carson's schedule as host of "The Tonight Show," which precedes Letterman at 11:30. Carson now takes both Monday and Tuesday nights off (except during ratings "sweeps" months, like February).

As a result, "Tonight Show" ratings are lower on Mondays and Tuesdays, and Letterman's fall, too.

"It would be great for us if Johnny was there five nights a week," says Letterman, who's only there four nights a week himself. "He's still the strength in that time period, and we're looking for any help we can get, and it's because of that I think that we're still on the air."

But he is not going to call Johnny up and scream at him. Nooooo.

"He's always been very very nice to me, and obviously very helpful, but he just scares me silly," Letterman says. "I can't relax with him because he's been such a presence in my life from the time I was in high school. I feel like I'm going to go to his house and I'm going to turn around and knock a vase over. And then of course Ed will have to come in and clean it up.

"Now Ed you can relax around. Ed's like watching a guy take a nap."

In fact, Letterman did go over to Johnny's house the other day, his palatial manse in Malibu, and did not knock over a vase. "It was nice," he says of the house. Nice? A sumptuous seaside spread that's an acre big indoors? "Nice for an Olympic venue is what I was going to say," Letterman adds.

Not all Letterman's NBC relationships are so chummy. Bryant Gumbel, the host of NBC's "Today" show, hasn't spoken to Dave since 1985, when Letterman interrupted a prime-time version of "Today" by standing in a window of Rockefeller Center with a bullhorn and shouting out things like, "I'm Larry Grossman, and I'm not wearing pants." Larry Grossman is the president of NBC News.

"Bryant just hates me," Letterman says. "He apparently is still angered by it. He really wanted to get into a fight or something."

"I was real angry at the time," says Gumbel, reached in his office. "If I'd had the opportunity to go after him physically, I would have. But I'm not that bad a grudge-holder. I don't wander the halls looking for David Letterman."

But Bryant, Dave says he wasn't allowed to come on the "Today" show and plug his special. "I'm not surprised," Gumbel says. "We do do some screening around here."

The problem was not that Dave disrupted the show but that he never called to apologize, Gumbel says. "Had I done the same kind of thing to somebody I liked or respected, and then heard they were upset about it, I'd have picked up the phone and said something like, 'Let me at least explain what I did,' or, 'Hey, I'm sorry.' That's never happened."

Letterman says Gumbel is mad at the wrong man, and that he should be mad at Steve Friedman, then executive producer of the "Today" show, because Friedman knew of the planned disruption and neglected to tell Gumbel. Friedman, now planning new shows for GTG Entertainment in New York, admitted yesterday that what happened was "partly my fault," a breakdown in communications between him and "Late Night" producer Morton.

Asked whether Gumbel should apologize to Letterman or the other way around, Friedman said, "I think they should both apologize to me, frankly."

Friedman said Letterman may have made matters worse by referring to the incident repeatedly on the air, and by playing old "Today" show promos with Gumbel in them and laughing at them. The one time Gumbel did the Letterman show, pre-incident, Letterman painted Gumbel's bare ankles orange. Lately he's been spreading the rumor that Gumbel has "a weight problem."

On a recent Letterman show, a "Top 10 list" of the least popular Campbell soups listed "Bryant Gumbo" as one of them. But Letterman says, "I think Bryant does a great job" and Gumbel says, "I'm not aggravated with David Letterman. If anyone wants to pin a label on it, the word is 'disappointed.' "

Gentlemen, gentlemen, must this continue? In the name of humanity -- Stop the Madness!

But no, it appears the madness will go on. Attempts to bind up the wound this week did not succeed. As for Letterman's crack about Gumbel gaining weight, this brings up another Letterman idiosyncrasy, a bizarre preoccupation with fat people, whom he appears to disdain -- en masse, as it were. He likes to spread the tale, for instance, that reporter Connie Chung married "a fat guy," and that her husband Maury Povich weighs 300 pounds.

"I have nothing against fat people," Letterman insists. "I must say, I watched that Connie Chung documentary about fat people, and I was always under the impression that fat people were fat because they would sit down in front of the TV with a bag of Kraft caramels and eat them just until they were gone.

"And as it turns out -- this is the lesson I learned from the lovely Connie -- that is not the case. And so as a result of that I have tried to enlighten myself regarding fat people. But even prior to that I never disliked fat people. So I'm working on this. I'm trying to grow here."

He doesn't promise never to do it again.

We wouldn't want him to promise never to do anything again.

Because Letterman's smart-alecky incorrigibility is part of his television persona. Part and parcel. More parcel than part, perhaps.

Nothing prevents outright cynical phonies from succeeding in television, but Letterman's far from that realm. Beneath the gags and the props and the wacky tics, one senses a genuine and self-deprecating straight arrow, one with a comic outlook that is authentically irreverent. Only David Letterman would listen patiently to George Will describing the contents of his latest tome, and outlining its thesis about the vicissitudes of the electoral college, and then ask him, "Do you seriously think anybody is going to buy this book?"

For Letterman's fans, whose ranks may swell at least a little after the anniversary special, "Late Night" remains a beacon, an oasis, a visit from the Good Humor truck. It's summer camp after the inmates have taken over. It's a full moon every night.