NEW YORK - Britney Spears has been ridiculed for everything from her 55-hour first marriage to backup-dancer second husband and her recent pantyless partying escapades. Now that she's entered rehab, though, the joke is over.

Pop singer Britney Spears is at a Sherman Oaks, Calif., tattoo parlor Friday night, Feb. 16, 2007. Spears was there to get a couple of tiny tattoos on her wrist, a pair of pink and red lips, according to KABC-TV. [AP]

This is a new frontier - even for Spears, whose well-documented gaffes and personal traumas have played out in excruciatingly public fashion, including photos published of Spears driving with her son Sean Preston on her lap and another time almost dropping him.

Spears' manager, Larry Rudolph, told People magazine's Web site on Tuesday that Spears had voluntarily checked herself into an undisclosed treatment facility.

"We ask that the media respect her privacy as well as those of her family and friends at this time," Rudolph was quoted as saying. He didn't disclose what substance she was trying to leave behind.

The 25-year-old pop star has been seen wearing skimpy outfits and partying heavily since filing for divorce from Kevin Federline in November, after two years of marriage.

On Friday night, Spears, the mother of two young sons, shaved her head bald. But that didn't send her into hiding as she was later seen wearing an inexpensive blond wig.

"You know, for these celebrities, it's really tough," New York-based psychiatrist Gail Saltz told The Associated Press. "They have no idea if anybody likes them for them. ... Everybody wants a piece. Everybody wants to make something off of her, get somewhere by her."

Spears' very public divorce isn't helping matters, said Saltz, who also suggested that Spears could be suffering from postpartum depression.

Federline's attorney had earlier confirmed that Spears and Federline would continue to share custody of 5-month-old Jayden James and 17-month-old Sean Preston this month, following terms of a January custody agreement.

"I'm a good father," Federline has said. "I love my kids and I'll always be here for (them)."

The Web site TMZ.com said Spears had entered an inpatient facility in Los Angeles after family members pressed her to check in.

Spears representatives didn't return calls from The Associated Press.

Entertainment TV shows and Web sites reported Friday that Spears had gone in and out of rehab that week.

"I think that she's finally listening to the people that are closest to her, and I think that she's probably going to stay in rehab for as long as it takes," Ruben Garay, who runs ThatOtherBlog, told the AP.

"You understand that this whole thing with shaving her head was kind of like the last straw, and that was really what led everyone (who) was already concerned about her (to) become, like, really concerned about her," said Garay, who recently closed his Spears' fan Web site, worldofbritney.com.

"She is a child star unraveling and is the perfect example of when a child star evolves into an adult and doesn't have someone telling them what to do, or giving them the right affirmative advice, and the right support," said Ian Drew, editor at large at Us Weekly magazine. "She basically started out very young in a competitive business."

Spears grabbed an electric shaver at a San Fernando Valley salon and shaved her head bald Friday night. Video on KABC-TV showed a newly shorn Spears with tiny tattoos on the back of her neck as she sat for a new tattoo - a pair of red-and-pink lips on her wrist.

Esther's Haircutting Studio, where Spears shaved her head, set up a Web site to auction off her hair and other items for a minimum price of $1 million, said J.T. Tognozzi, who owns the salon with his wife.

"This is it, the opportunity of a lifetime," according to BuyBritneysHair.com. The winning bidder gets Spears' dark, knotty hair extensions, the clipper she used to cut them off, the Red Bull she drank at the salon and her cigarette lighter.

"This girl is out of control," Joy Behar, a co-host on ABC's "The View," said Monday. "And, she's in a lot of trouble. A lot of people feel this is self-mutilation."

Craig Ferguson, host of CBS' "The Late Late Show," said that after seeing photos of Spears' shaved head, he reconsidered making jokes at the expense of the "vulnerable."

"For me, comedy should have a certain amount of joy in it," he told viewers Monday. "It should be about attacking the powerful - the politicians, the Trumps, the blowhards - going after them. We shouldn't be attacking the vulnerable."