Appearing by satellite Thursday for Leno's "Ten at Ten" question-and-answer segment, the ABC late-night host was asked to relate his best prank ever. Kimmel replied that he told a guy five years ago that he'd give him his show, and "then I took it back."
It was a thinly veiled reference to Leno's agreement in 2004 to surrender "The Tonight Show" to O'Brien in 2009, after 17 years as host.
NBC, which is ending Leno's prime-time show, wants to return him to 11:30 p.m. EST by bumping O'Brien and "Tonight" to midnight, a plan O'Brien has rejected. The network was in talks with both hosts.
Kimmel also joked that Leno had "$800 million, for God's sake," and advised him to leave other hosts' shows alone.
Leno might have known what to expect from his guest, who'd already made pointed fun of him on Tuesday's "Jimmy Kimmel Live." Wearing an exaggerated fake chin to play Leno, Kimmel joked that he was taking over all late-night shows, including Kimmel's, and announced that ABC stands for "Always Bump Conan."
O'Brien did some pummeling of his own Thursday, in what is becoming a pattern for him and Leno.
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"Hi, I'm Conan O'Brien, NBC's 'Employee of the Month,'" he said in his monologue. "There's a rumor that NBC is so upset with me, they want to keep me off the air for 3 years. My response to that is, if NBC doesn't want people to see me, just leave me on NBC."
Leno delivered a punch in his monologue: "Welcome to the new show, 'I'm a Celebrity, Get Me Off NBC!'" he said.
Meanwhile, NBC looked to the future by announcing that new and veteran NBC dramas and a comedy produced by Jerry Seinfeld will take over the bulk of the prime-time slots soon to be vacated by Jay Leno.
The network said Thursday that the freshman drama "Parenthood" and the relocated "Law & Order" and "Law & Order: Special Victim Units" will fill three slots. Another will go to the comedy panel series "The Marriage Ref" from Seinfeld.
"Dateline NBC" will fill another 10 p.m. EST slot.
The new lineup will debut after NBC's coverage of the Winter Olympics from Feb. 12-28.
For now, though, dramas can claim to be winners in the schedule shake up, reclaiming most of the slots lost when NBC decided to experiment with Leno's show as a cheaper alternative.
"Law & Order" will return after the Games on Monday, March 1, with a two-hour episode airing at 9 p.m. EST. The show, in its 20th season, will begin airing regularly at 10 p.m. the following week.
"Parenthood," with an ensemble cast including Lauren Graham and Peter Krause, will debut 10 p.m. EST Tuesday, March 2.
The "Special Victims Unit" member of the "Law & Order" family will air in the final prime-time slot on Wednesdays, starting March 3.
The other new 10 p.m. show, "The Marriage Ref" with comedian Tom Papa, will air Thursdays beginning March 4 after a Feb. 28 sneak peek following the closing ceremony of the Olympics. "Dateline NBC" will air 9-11 p.m. EST Fridays starting March 5.
Also bowing after the Olympics are "Who Do You Think You Are?" - a look at the family history of celebrities, 8 p.m. EST Friday, March 5, and "Minute to Win It," a game show with TV food personality Guy Fieri, 8 p.m. EST Sunday, March 14.