Jack Osbourne, the 17-year-old party hearty son of Ozzy and Sharon Osbourne, has checked into a Southern California rehab center, reports TV's "Entertainment Tonight."

Staff at the private Las Encinas clinic in Pasadena (right outside Los Angeles), where the Osbournes' only son checked in on Saturday night, have refused to say what Jack is being treated for.

"ET" reports that the young Osbourne was accompanied by his parents when he checked in as patient 021042 and put on a white hospital gown. Fees at Las Encinas are said to start at $800 a night.

According to Britain's Daily Mirror, Sharon, 50, told the tabloid from her California home: "I can't talk about it. He is my baby, he will always be my baby."

Pressed on the matter of what Jack was doing in Las Encinas, she said: "What do you think? Why do people go into rehab clinics?"

Ozzy, 54, has made no secret of his heavy use of amphetamines, pot and cocaine in his younger Black Sabbath days, though he reputedly has been clean since 1991 -- thanks to an intervention by Sharon.

Jack has been equally candid about smoking pot, and, when asked of his future goals, he once declared at a press conference, "I want to be on the cover of High Times magazine someday."

Striking a more serious note in another interview, Jack admitted that he began having difficulty dealing with his fame and suffered insomnia and depression when the MTV show, "The Osbournes," first went on the air.

"You lose your anonymity," he said. "I can't enjoy myself when I go out anymore. I've been hit, I've been slapped, people drive by screaming they hate me."