No joke: Jim Carrey is packing on the pounds to play "Curly" in the upcoming movie version of The Three Stooges.

"He's gaining weight," girlfriend Jenny McCatrthy says on Tuesday's Ellen DeGeneres Show. "He's going to gain 50 pounds to play Curly."

She says he's been "basically enjoying his desserts."

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"He's just having so much fun with it," she adds. "I have no idea what it's going to be like when he hits near 300 pounds. But, you know, he's in his comedy. He works hard at it."

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"People are going to be blown away when they see him," she says.

Does it ever get to be too much having to hear him walk around the house doing Curly impersonations all day?

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McCarthy puts it this way: "You have to be very much in love with him [to tolerate it]."

Last month, Variety reported that Sean Penn will play Larry and that negotiations were underway with Benicio Del Toro to play Moe.

The film is not a biopic, but rather a comedy built around the antics of the three characters.

Production will begin in early fall for a 2010 release.