David Letterman David Letterman

When you're No. 1, you usually ignore the competition. Not David Letterman.

The host of CBS' late-night show used his Wednesday monologue to talk not only about NBC's Conan O'Brien — and Jay Leno — but also ABC's Jimmy Kimmel.

Taking a page from the playbook of mentor Johnny Carson, he got into it by using the weather.

"Cold outside, isn't it lousy cold outside today?," he said on Late Show with David Letterman. "Whoo! You know, they say, from the weather bureau, they say it's caused by an arctic chill between Jay and Conan."

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He went into the hubbub of the proposed timeslot shakeup, saying it even affected Carson Daly — who's "now's got to get a show in Mexico or something."

"It'll make you dizzy. So Conan O'Brien says yesterday, 'Well, I'm not doing The Tonight Show at 12:05,' and you think about it, well, he's right. The Tonight Show, that would, you know, that's the next day. It's not really — it's like The Day After or, what it used to be, The Tomorrow Show. So he said, 'Forget it, I'm not doing the show at 12:05.' Well, NBC went back to him, and they decided to sweeten the deal — they offered him 12:04." 

Conan O'Brien rejects Tonight Show move to 12:05

Letterman, of course, once coveted taking over Tonight from Carson but NBC in the early '90s chose to give it to Leno over him — provoking his move to CBS. So in discussing O'Brien's possible departure to another network, he joked: "Where'd he get an idea like that?"

"The whole idea of this was NBC wanted to get a thing going whereby they wouldn't make the same mistake they made when Johnny quit and retired, that there were a lot of bad feelings. They wanted to avoid causing more bad feelings — well, mission accomplished."

Letterman recalled that when Carson retired, he never came back — no specials, nothing (well, except for a cameo or two on Letterman's program). "I miss Johnny Carson — I mean, by God, when Johnny quit, he quit."

Jimmy Kimmel does scathing imitation of Jay Leno on ABC show

He also said he thought Kimmel doing his entire ABC show as Leno "was good."

"Jimmy Kimmel was so convincing as Leno, today, NBC canceled him," Letterman said.

"I want to tell you something, I have not been this entertained by NBC since Balloon Boy threw up on the Today show."

Letterman further poked fun at how NBC has to scramble now to fill five hours of prime-time programming: "And, you know, they're up against it — first week, I'll give you an example of what they're going to do — first week, YouTube cat videos, five hours, across the board."