Publicist Susan Patricola says Hartnett was transported by ambulance at 1:45 a.m. Monday to Cedars-Sinai Medical Center.
She says in a statement that he "is under observation but is resting comfortably."
Patricola says Hartnett had a flare-up of a gastrointestinal issue that affected him while he was starring in an onstage production of "Rain Man" in London last year.
No other details have been provided.
Hartnett has appeared in the films "Black Hawk Down," ''Sin City," ''40 Days and 40 Nights" and "The Black Dahlia."
Copyright 2009 The Associated Press.