The Mask star was wheeled out from behind a curtain, wearing a red tracksuit and headband, and insisted he had to conduct his chat with late-night host Jimmy Fallon while he was jogging.
He breathlessly explained, "I gotta get back in shape after Christmas... I think it's very important that I stay my very best for the fans; I'm hoping to get down to about 150, 155 (pounds)... I'm 190."
And he joked about hitting his weight goal in two weeks: "I've lost about 60 pounds in a weekend. That's Hollywood, man... I had a disease though... It eats your skin; it's fantastic."
The comedian told the chuckling Fallon he likes to stay active - so people don't think he's dead.
He added, "I've been buried three times, buried alive. I was cremated once - that was tough to come back from."
Carrey then took a break to pretend to pee into a mug, before returning to the treadmill to continue running.
He concluded the odd chat by playing a guitar as he jogged.
Carrey wasn't the only funnyman getting fit while he promoted new movies on U.S. TV in the last 24 hours - Seth Rogen chatted to Today show hosts Kathie Lee Gifford and Hoda Kotb about his new film The Green Hornet while pedalling on an exercise bike.
Meanwhile, Ryan Gosling tested his reputation as a serious actor by promoting new film Blue Valentine on Ellen DeGeneres' daytime chat show from an exercise bike.
When the host teased him about his rumoured real-life romance with Blue Valentine co-star Michelle Williams, Gosling started to pedal faster, explaining, "I'm trying to get away from this conversation."