Jessica Simpson paid a late-night visit to ... jail!
The singer, 29, stopped by the L.A. County Sheriff's Department in West Hollywood early Friday morning to try to bail out her friend, Usmagazine.com confirms.
Her pal and longtime makeup artist, Mary Phillips, was arrested at 1:40 a.m. for being drunk in public after police received a disturbing the peace call at the Sunset Tower Hotel, a law enforcement source tells Us.
Simpson, however, was turned away when she attempted to bail her friend, 30, out.
"She was arrested for being drunk in public, and she was not released to Jessica because you can't release someone who has not sobered up," Deputy Aura Sierra tells Us. "They're going to release her a little later when she's sobered up. I don't know what time that will be. It could be a minimum of 6 to probably 8 hours."
Hours earlier, Simpson had posted a photo on Twitter of herself, hairstylist BFF Ken Paves and Phillips -- who worked with Simpson on The Dukes of Hazzard and Employee of the Month and prepped her for her 2002 wedding to Nick Lachey.
She said that she was "sizzlin' with my peeps, pretty boy KP and Lil' Cholita [Phillips] ... look out yo!!"
Simpson's rep had no comment.