How does David Letterman feel about his new ratings success? How does David Letterman feel about being on the Phil Donahue show? How does David Letterman feel about show business?

Aw, who cares how David Letterman feels about anything?

You care, that's who cares, because you're a caring person. A card-carrying caring person. You want to know David Letterman's innermost thoughts, especially now that his show, "Late Night With David Letterman," is enjoying vastly improved Nielsen numbers, and just as it celebrates its third anniversary, too, with a 90-minute NBC special at 11:30 tonight on Channel 4.

Though not screened for TV critics (the very idea!), the program could very well turn out to be AN ALL-TIME KOMEDY KLASSIC!!! With plenty of those hilarious capers and antics we have all learned to expect from Dave. Now he graduates from cult item to Really Big Deal, meaning, Nielsen's computer has been burping out the highest ratings of his show's career.

"This ratings thing is very mystifying to me," says an always worried Letterman, the Bob Hope of the hip, from his cluttered Manhattan office ("the Hope of the hip" -- yes, I think that will stick). Letterman was recently told by NBC Entertainment president Brandon Tartikoff that his ratings are up 25 percent over last year. "I don't know what the hell happened," Letterman says. "We're not doing anything different on the show. Actually, I thought we were doing better shows in some ways in our first and second years. Maybe we've just worn people down. They just say 'Oh all right, oh God, we'll watch your damn show.' "

Since he doesn't know what made the ratings go up, he can't do more of it. At any rate, his time has come, even though his time is 12:30 in the morning. More NBC affiliates than ever are carrying the show, and Baltimore's WMAR-TV recently decided to pick up Letterman even though it does not air Carson.

Letterman is hardly the type to let good news go to his head, even good news about ratings. "They'll probably just go right back down again," he sulks.

Unpretentious, undemonstrative and unimpressed with his own success, Letterman has carved out comedy territory that is all his own. But that may be because no one else wants it. He drops watermelons off 11-story buildings, crushes Smurfs under steamrollers, rails against the injustices perpetrated by his cable TV system, and threatens to sue guests who don't show up on his program for $1.5 million. On occasion he commandeers a portable camera and invades the local WNBC-TV news show, "Live at Five," while it's on the air in a studio just across the hall from his.

Last week, he tried using his influence, and a generous bribe, to get the show's gentlemanly resident lump, Larry "Bud" Melman, his own star on Hollywood Boulevard. All in the noble cause of skewering sacred showbiz cows, and hams. "The longer I'm alive, the more I hate show business," David Letterman says. "Every time I go back to L.A., which I just did, I say to myself, 'This is the silliest, stupidest way in the world to make a living.' "

That look of ingratiating boyish guilt that comes from earning more than $1 million a year to be wacky on TV is part of what endears Letterman to a baby-boomer constituency that has endured wave after wave of show-biz phoniness and a lifetime of tacky television tripe.

Letterman really was uncomfortable sitting in the celebrity hot seat at the "Donahue" show, taped earlier in the week and shown yesterday. Donahue, who's been frequently ridiculed on "Late Night," tried to get even with a petty series of aspiring embarrassments, old videotape of Letterman as a weatherman and visitations from a high school teacher and a fraternity brother.

"Boy, was it strange," Letterman said after the taping. "Phil used to get such great audiences in Chicago, but what he's gettin' here are flood victims. I knew it would be a long hour, and boy was it ever. It was so bizarre, my heart sank. I just wanted to go home."

Of course to a viewer at home it just seemed like a pretty funny and agreeable show, with Donahue playing the fool, and typecast at that.

One of Letterman's frequent guests, Jay Leno, comedian and master railer at the injustices of life, says of his friend Letterman, to whose home he has incidentally never been invited for dinner, "David knows exactly what he does. Like, the funniest thing in the world is when I see David 'act' on his show." Leno smiles broadly. "He'll do a bit, and say something like, 'All right, put the gun down.' I get hysterical! I call him up and say with heavy sarcasm , 'Wonderful acting job!' 'Cause, see, a lot of people in show business don't know their limitations, and they always try to go over them, and sing or something, and it looks clumsy, but David knows exactly what he does."

Crazily inventive and yet studiously cautious, Letterman doesn't like to venture outside his chosen turf. He declined an invitation to be among the stars at last week's inaugural gala, pleading "a longstanding family commitment" as a rather transparent excuse. When, after a recent "Tonight Show" appearance, Johnny Carson asked Dave if he'd like to come over to Johnny's new Malibu beach house (it's one acre inside) and play tennis, Dave freaked. "I realized I was terrified of actually spending time with him," he says. He never called Johnny up.

For tonight's third anniversary program, Letterman and company staged a "Late Night Baby" contest between two New York hospitals. The first baby born during the taping of the program would be declared the "Late Night Baby" and, says Letterman, become "the wealthiest, most powerful child in the world." Letterman was asked what would happen if the mother didn't want this particular honor. "Then we'll flash her a twenty or two," Letterman said confidently. "We'll change her mind."

Letterman's targets often include the pandering inanity of television. His writing staff and producers are untiringly inventive and cheeky (they could beat up the "Saturday Night Live" writing staff any night of the week) but Letterman helps keep the humor smart-alecky without being nasty, except in cases where there just isn't any choice. Leno admires the way Letterman refrains from "sneering sexist humor" about such well-trammeled comic topics as Dolly Parton's breasts. "I like the show because even if they do something that doesn't work, there's never a cheap shot to try and save it," Leno says.

Yes, Letterman does have his standards. Asked why he refused to try on a wig offered him one night by wig-toting guest Eva Gabor, Letterman says, "We have kind of a rule of thumb on the show: I don't wear funny headgear. I feel like I look like a jerk to begin with." Looks may be deceiving. David Letterman presides over the brightest, sanest loony bin in all of television.