The troubled actor was arrested early Tuesday morning in the Los Angeles suburb of Culver City on suspicion of being under the influence of a controlled substance.
Exactly what kind of controlled substance was not immediately known. But the 36-year-old actor was picked up by Culver City police at about 12:10 a.m. while walking on the 12000 block of Washington Boulevard.
"One of our patrol officers observed him and began an investigation after seeing him in an alley," police Lt. Dave Tankenson said in a statement. "During the investigation, the officer determined that [Downey] was showing symptoms of being under the influence of a controlled substance."
Downey was booked on the misdemeanor drug charge, given a urine test and then released to his parole agent, Tankenson said. According to a statement released by his publicist, the actor has voluntarily checked himself into an undisclosed rehab facility. He is due in court for arraignment on May 4.
After learning of the arrest, the producers of Ally McBeal dropped Downey from his Golden Globe-winning role as Ally's love interest, Larry.
"Robert is a unique talent and a very special person, and we wish him the best and hope for a full recovery. We are wrapping up the stories on the final few episodes of Ally McBeal for the season without him," show mastermind David E. Kelley said in a statement.
Sources say Kelley will have to rewrite the remaining episodes for this season, which would have featured Downey's character and Calista Flockhart's Ally debating whether to continue their relationship. It is unclear whether Downey would be invited back next season.
Until now, Kelley had publicly supported Downey through his legal woes and battle to stay sober, even extending the actor's contract.
Downey was due to appear Tuesday night at the Virgin Megastore in Hollywood, where he and Ally songstress Vonda Shepard were expected to sign copies of the new Ally McBeal disc, For Once in My Life, on which they duet. The album release party was canceled.
Tuesday's arrest marks the latest criminal run-in for the actor, whose release from prison last year--followed by a renewed acting career and subsequent battles with addiction--have been watched closely.
The actor is still due in an Indio, California, court next Monday for a pretrial hearing stemming from his Thanksgiving weekend drug bust in Palm Springs.
If convicted, Downey could face another four years in prison. Meanwhile, his lawyers are continuing to wrangle in court and are now challenging the legality of the hotel-room search by Palm Springs police last November. The actor was arrested after police--responding to an anonymous 911 call saying Downey had guns and drugs--allegedly found cocaine and Valium in his room at the Merv Griffin Resort.
But Downey's attorneys have filed motions to quash the search warrant and suppress evidence, saying police illegally obtained the cocaine and Valium when they entered the room. His lawyer, Robert Waters, contends the anonymous 911 call was not enough evidence to warrant a search.
His lawyers also want to suppress Downey's statement that he allowed officers to search his room, and they're also challenging the legality of his Valium possession charge. Downey's lawyers contend the Valium was not illegal.
(updated 7:30 p.m. PT)