Mayfield, 39, was randomly tested last weekend at Richmond. NASCAR received the results today at noon (et). He became the first driver to be suspended since NASCAR's revised drug policy went into effect this season.

"As both a team owner and a driver in the NASCAR Sprint Cup Series, I have immense respect for the enforcement policies NASCAR has in place," Mayfield said in a statement. "In my case, I believe that the combination of a prescribed medicine and an over the counter medicine reacted together and resulted in a positive drug test."

The driver and owner of the No.41 Toyota finished 35th at Richmond, but failed to qualify for the Southern 500 at Darlington Raceway on Friday.

Dr. David L. Black, an internationally recognized forensic toxicologist and founder of Aegis Sciences Corp., oversees NASCAR's drug policy.

"My doctor and I are working with both Dr. Black and NASCAR to resolve this matter," Mayfield added.

Tony Martin, a crew member from Tony Raines' No.34 Front Row Motorsports team in Sprint Cup, and Ben Williams, a Nationwide crewman for Roush Fenway Racing's No.16 team, were also suspended indefinitely for violating the policy.

"NASCAR has one of the toughest substance abuse policies in all of sports," Jim Hunter, NASCAR's vice president of corporate communications, said. "It's all about safety and competition. And a positive result means an indefinite suspension from all competition."

NASCAR does not have an appeal process for its policy.

"(Mayfield) is suspended as an owner and a driver," Hunter added. "His team could be operated by somebody else, but Jeremy will not be allowed to participate."

Mayfield Motorsports will continue to compete in Sprint Cup, with an owner and driver named presumably in time for next weekend's all-star race at Lowe's Motor Speedway.

"Mayfield Motorsports remains committed to the NASCAR Sprint Cup Series and the organization will announce an interim owner and a temporary replacement driver early next week," Mayfield said. "Those roles will commence immediately beginning with next week's Sprint Open and continuing through Charlotte and beyond."

NASCAR revealed its new substance abuse policy last September, with all drivers and crew members tested at the beginning of the year and subject to random tests throughout the season.

The policy for testing someone who is of "reasonable suspicion" remained in effect.

Penalties for violating the policy remained the same, with immediate suspension from competition. While it is possible that a competitor could receive a lifetime ban for just one violation, a third violation will automatically result in a lifetime ban.

Mayfield has five career Sprint Cup victories. His most recent win came in August 2005 at Michigan while driving a Dodge for car owner Ray Evernham.

This year, Mayfield formed his own team and qualified for five races, including the Daytona 500. He is currently 44th in points.