Unlike many comedians, Jay Leno isn't easily pigeonholed.

But one label fits his 6-foot frame and his flip frame of reference: "The Nice Comedian."Not to say that he pulls his punchlines. Consider his monologue one recent Monday when he filled in for Johnny Carson on NBC's "The Tonight Show."

The show's permanent guest host once again ripped Exxon but did (nicely) give the beleaguered oil company credit for efficiency:

"As soon as the tanker crashed, Exxon officials were in communication with Exxon workers all over the country. And by the next morning" - he snaps his fingers - "gas was up 50 cents a gallon all over the place."

Or on House speaker Jim Wright:

"He wrote a self-aggrandizing book about himself and charged too much for it. Is that a crime now? I mean, if it is, I think Shirley Mac-Laine will probably get the electric chair.

"And in the self-righteous department," he noted, "Domino's Pizza is dropping their advertising from `Saturday Night Live' because they say the show does not promote family values. Hey, what kind of family values does Domino's represent? Let's be honest: `Hey Ma, you gonna cook dinner tonight?' `Shut up and call Domino's if you want something to eat!' "

Not really nice, but not crude, racist or sexist. And though this is network TV, Leno's not much dirtier live.

"So much comedy now is gay-bashing, women-bashing. . . . It's all a lot of negative stuff," he says. "I try to not be negative, because my life is not particularly negative."

Indeed, things continue to go, well, nicely for the 38-year-old comic. He stands in for Carson on Mondays and during Carson's frequent vacations - a job he's held since 1986.

And the 15-year show business veteran - happily married for eight years to his wife, Mavis - still manages to jet around to some 250 live shows every 365 days.

He doesn't have to, since he's making about $3 million a year now, but knowing he won't stay hot forever makes it hard to turn down a job. "What else am I doing on a Tuesday that's worth $10,000?"

Hard work and humbleness have gotten him this far without many enemies - you just don't hear or read much dirt on Leno.

Maybe the only time he does get dirty is when he's working on his classic collection of 19 motorcycles, a three-wheeler and eight cars.

"I like working with my hands," says Leno, who was employed as a mechanic while growing up in Boston. "Most of my friends are mechanics and machinists," whom he prefers to Hollywood hoopla.

He doesn't want to do films: "Too much like a day job."

He's also turned down offers to do his own talk show. As far as his role on "The Tonight Show" goes: "Would I take it if (Carson) left? Sure. . . .

"People always say, `What do you want to be doing in five years?' Like I should switch to modern dance."

He's happy doing what he's doing. "I mean, you go around and tell jokes. Nobody gets cancer of the pancreas."