Mayfield tested positive for a banned substance last weekend at Richmond International Raceway.
"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," Mayfield said in a statement. "My doctor and I are working with both Dr. (David) Black and NASCAR to resolve this matter."
Black is the CEO of Aegis Sciences Corp. in Nashville, Tenn., which runs NASCAR's testing program.
NASCAR spokesman Jim Hunter would not reveal what banned substance Mayfield used, but Hunter said it was not an alcohol-related offense.
"There is no place for substance abuse in our sport," Hunter said.
NASCAR also suspended two crew members for failed tests at Richmond.
Tony Martin, a crew member for the car John Andretti drove last weekend at Richmond, and Ben Williams, a crew member for the Nationwide Series car Matt Kenseth drove last weekend, were both suspended indefinitely.
Mayfield, who is driving a car this season he owns himself, failed to qualify for Saturday night's Sprint Cup race at Darlington Speedway.
Four-time series champion Jeff Gordon said Mayfield's suspension shows how serious NASCAR is about its new testing policy.
"I think it's very unfortunate for Jeremy, but it's going to send a big message through this garage area to make sure we are treating this sport as professionally as it deserves to be," Gordon said. "There are too many guys out there and you know somebody is going to make a mistake. We've heard of other instances outside of the Cup series. I think it's just very unfortunate for the sport, for Jeremy, for everybody.
"It definitely puts a black eye on it. We'll take the blows and move on."
NASCAR said Mayfield was randomly tested last Friday in Richmond. The Aegis lab discovered the positive "A" sample Tuesday and notified Mayfield. Two days later, the lab told NASCAR of the failed test.
Mayfield, who participated in both of Friday's practice sessions at Darlington, asked Friday for his backup "B" sample to be tested. That, too, came back positive, and he was told by Aegis officials Saturday afternoon.
Black said he spoke with Mayfield, who can return to NASCAR only after he completes a "path for reinstatement" that's tailored to each individual. The process, which can include rehab, varies depending on the substance.
The suspension, which cannot be appealed, applies to Mayfield's roles as owner and driver of the No. 41 Toyota. Although the car can race next week at Lowe's Motor Speedway with another driver, Hunter said it cannot be entered with Mayfield as the owner.
The 39-year-old driver said in his statement that an interim owner and a temporary replacement driver would be announced early next week.
Andretti, who finished 32nd in last week's race at Richmond, said he's not worried that the driver next to him might be driving impaired and applauded NASCAR's tougher drug policy.
"I think it's a great thing that they (NASCAR) do," Andretti said from Indianapolis, where he's preparing for the Indy 500 later this month. "And whoever they catch and confirm, so there's no mistake, shame on them."
Just days after the Daytona 500, one of Mayfield's crew members became the first person punished under NASCAR's new drug policy for a failed test. Mayfield fired Paul Chodora after he was suspended.
Mayfield, a two-time qualifier for the Chase for the championship, has five Cup victories in 433 career starts, but none since 2005 at Michigan. He was fired by Evernham Motorsports in late 2006 and bounced around until this season, when he formed Mayfield Motorsports.
He threw the team together in less than a month but made headlines as the underdog who raced his way into the season-opening Daytona 500. But he made just four of the next 10 races, and is currently 44th in the Cup standings.
NASCAR announced a new, tougher drug policy last September. The guidelines were strengthened in part because of former Truck Series driver Aaron Fike's admission that he had used heroin -- even on days he raced.
Under the new rules, all drivers and crew members were tested before the season began. Random tests are scheduled throughout the year, and at least four drivers are tested each weekend. Hunter said the drivers are selected through an automated computer program.
------
AP Auto Racing Writer Jenna Fryer in Charlotte, N.C., and Associated Press Sports Writer Mike Marot in Indianapolis contributed to this report.
