Williams' agent, Leigh Steinberg, said the running back, who will turn 30 in May, had sent a letter asking that he be allowed to return to the Miami Dolphins.

The NFL confirmed that Williams had sought reinstatement, for which he will be eligible after April 27, when he will have been suspended for a year. Dolphins spokesman Harvey Greene said the team had no comment because Williams is not yet eligible for reinstatement.

Williams, a former Heisman Trophy winner at Texas, was originally drafted by New Orleans when then-coach Mike Ditka gave up all his picks in the 1999 draft to get his rights. He was traded to Miami for two first-round draft picks after the 2001 season and rushed for 1,853 yards for the Dolphins in 2002 and 1,372 the next season.

But he abruptly retired shortly before training camp in 2004, then returned in 2005 to back up rookie Ronnie Brown. Williams rushed for 743 yards and a 4.4 yard average that year.

The suspension, handed down last year by former commissioner Paul Tagliabue, was Williams' fourth.

Williams' previous positive drug tests were for marijuana, which he acknowledged using. The latest test apparently involved a substance other than marijuana and may have been related to his interest in holistic medicine.

Steinberg said Williams has been teaching yoga in California since returning from Canada last December.

''He's told me that his interest in eastern philosophy has overcome his desire for mind-altering substances and has given him another outlet,'' Steinberg said. ''He's in outstanding shape and he hasn't taken a lot of hits for a running back. I think he's still at the top of his game.''

During the 2005 season, Williams laughingly described himself as weird and compared his career to a roller coaster. But he also dispelled his reputation as an aloof, selfish pothead, winning praise from teammates and winning the south Florida media's annual postseason Good Guy Award, given to the player most cooperative with reporters.

Williams has gained 7,097 yards in 1,757 carries during his NFL career and rushed for 47 touchdowns.