Is Michael Phelps Training for the 2016 Rio De Janeiro Summer Olympics?

Michael Phelps has rejoined the U.S. drug testing program -- a requirement to enter upcoming international competitions -- sparking rumors that the winningest athlete in Olympic history is planning a comeback for the 2016 Summer Olympics in Rio De Janeiro, according to a WBAL-TV story.

The Towson native told The Associated Press on Thursday that "nothing is set in stone" though clearly he has enjoyed getting back into shape -- he's down about 15 pounds -- and working out with his former team at the North Baltimore Aquatic Club, reports ESPN.

"If I decide to keep going and swim again, then I'll compete," Phelps told the AP, according to WBAL's story. "If I don't," he added, letting out a big laugh, "I guess I'll re-retire. Just don't compare me to Brett Favre."

His former coach and close friend, Bob Bowman, said Phelps re-entered the drug-testing program near the end of the second quarter, reports ESPN. Phelps would be eligible to compete again in March, according to Bowman.

That coincides with Bowman telling The Associated Press last month the swimming great had resumed occasional training at the North Baltimore Aquatic Club. Bowman, however, characterized it more as an effort to get in shape than launch a comeback, says The Washington Post.

By subjecting himself to drug testing, Phelps sets himself up to begin competing again by next summer if he chooses, the Post's website says. That would give him time to get in peak condition for the next major meet, the 2015 world championships in Russia, where nearly all of the top swimmers are expected to take part as a steppingstone to the Rio Games the following summer.

Each week, Phelps told the AP, he's working out a couple of days in the pool, a couple of days in the weight room, and one or two days on his core training.

"I just think he's in a place where he's feeling good about swimming," Bowman told the wire service. "To be perfectly honest, he's not anywhere near being able to compete in a meet or anything like that."

Phelps is the winningest and most decorated athlete in Olympic history.

He captured 18 gold medals and 22 medals overall at the last three Summer Games, shattering the previous marks. He is best known for breaking Mark Spitz's record for a single Olympics by winning eight gold medals in Beijing in 2008.

Phelps retired at age 27 after winning six more medals at the 2012 London Olympics. He had long said his goal was to retire from swimming before he turned 30, the AP reported.