What if fat-burning workout consisted of popping a pill?

Scientists have discovered what could be the ultimate workout for couch potatoes: exercise in a pill.

Researchers at the Salk Institute in San Diego reported that they have found two drugs that do wonders for the athletic endurance of couch-potato mice. One drug, known as Aicar, increased the mice's endurance on a treadmill by 44 percent after just four weeks of treatment.

A second drug, GW1516, supercharged the mice to a 75 percent increase in endurance, but had to be combined with exercise to have any effect.

"It's a little bit like a free lunch without the calories," said Dr. Ronald Evans, leader of the Salk group.

The results, Evans said, seem reasonably likely to apply to people, who control muscle tone with the same underlying genes as do mice. If the drugs work and prove to be safe, they could be useful in a wide range of settings.

The drug, according to the researchers, changed the physical composition of muscle, essentially transforming the tissue from sugar-burning fast-twitch fibers to fat-burning slow-twitch ones — the same change that occurs in distance runners and cyclists through training.

The researchers said the drug's fat-burning ability also could help reduce weight, ward off diabetes and prevent heart disease — the benefits of daily aerobic activity without the perspiration.

"It's an amazing piece of pharmacology," said David Mangelsdorf, a pharmacologist at University of Texas Southwestern Medical Center in Dallas, who was not connected with the research. "You're getting the benefits of exercise without having to do any work."

It is unknown if the drug has any benefit for athletes who actually work out — or any human for that matter, because the research, so far, has only involved mice.

"The mouse doctors and cell biologists are of course quite enthusiastic about these things, but the human doctors are a little more reticent," said Dr. Benjamin Levine, a cardiologist who leads the Institute for Exercise and Environmental Medicine at Presbyterian Hospital of Dallas, and who was not involved in the study.

But Evans said he already has been contacted by dozens of athletes and overweight people who have heard about his research.

Evans said he has notified world anti-doping officials, who are scrambling to implement a test for it before the Beijing Olympics start next week.

The compound, which is naturally produced in tiny amounts in human muscle cells and has been studied for decades, is readily available through Web sites that cater to researchers. One site was offering it for $120 a gram.

Evans predicted that in the wake of his study, published Thursday in the journal Cell, it will "fly off the shelves."

With more research, he said, the drug might one day be used as a treatment for muscle wasting, obesity and as a means of allowing bedridden patients to reap the benefits of exercise.

The drug has been tested in humans for a variety of conditions related to the heart and repeatedly passed basic safety tests.

"It was found to be a quite safe drug, at least at the doses we were using," said chemist Paul Laikind, who patented the compound in the 1980s and began testing it as a means of preserving blood flow to the heart during surgery.

The compound is now owned by drugmaker Schering-Plough, which is trying to develop the compound as an intravenous infusion for the prevention of ischemia-reperfusion injury, a complication of bypass surgery.

Information from The New York Times is included in this report.