BACK IN THE EARLY 1960s, when the architect Louis Kahn designed the airy layout of the Salk Institute—a collection of stark concrete towers aligned like teetering dominoes on a Pacific Ocean bluff in La Jolla, California—he oriented the buildings so that robust sea breezes would waft through the upper floors. But as I descend four flights of stairs to enter a sprawling subterranean lab, the sweet ocean air turns sour. Researchers at Salk are conducting cutting-edge experiments in genetics, biology, neuroscience, and human physiology. At the core of this futuristic work are 6,000 old-fashioned, defecating rodents, stacked in shoebox-size plastic cages, creating an odor far too potent for Kahn's ingenious ventilation scheme to handle.

Despite the funk, the facility is meticulous. Wearing powder-blue scrubs, a surgical mask, a bouffant cap, and cloth shoe covers, I enter through a sterile clean room closed off between double doors. A whitewashed hallway adjoins various smaller labs, where mice are being injected with performance-enhancing compounds and forced to sprint on tiny treadmills. Others have had bits of their DNA reprogrammed to make them better runners. There are paunchy mice gorging on high-fat diets and svelte mice getting low-cal meals. Hunched over a metal table, a technician sorts through a squirming posse, plucking out prime studs for breeding and banishing aggressive males to solitary confinement. Mice are sacrificed and their muscles examined. Blood is sampled, hearts are inspected, kidneys and livers prodded.

This busy little world is the multi-million-dollar endeavor of Ron Evans, a 61-year-old molecular and developmental biologist who's trying to crack the code of human endurance. With help from a team of 35 scientists, Evans has an ambitious goal: to develop the first-ever performance-enhancing drug that can radically boost physical endurance in humans.

The "exercise in a pill" project began during the summer of 2007, when Evans made a stunning announcement. While investigating obesity, he stumbled upon a genetic switch that unexpectedly turned his lab rodents into super-athletes. In August 2008, Evans published the findings in Cell, a prestigious scientific journal, claiming that in some cases his augmented mice could run 90 percent farther than ordinary critters. By comparison, it's considered extraordinary when a human athlete's performance jumps by only 3 percent. Evans's breakthrough would be like transforming a dawdling weekend jogger into an Ironman contender overnight. And, as Evans assures me, "This wouldn't require you to actually exercise muscle to gain a benefit."

In the now famous Cell paper, Evans and his co-authors—a collaborative multinational team based at research institutes in California, Massachusetts, and South Korea—confidently announced that they had found a way "to enhance training adaptation or even to increase endurance without exercise." Physiologists who'd spent their careers deconstructing the sophisticated mechanics of exercise and its numerous benefits were skeptical, dismissing the notion of pill-popping your daily workout as ludicrous.

But that didn't stop every major media outlet—including the big four networks, cable news channels, The New York Times, and The Wall Street Journal—from declaring the breakthrough a "couch potato's dream." Nova scienceNow, a PBS program, interviewed Evans, who said that "the benefit of exercise alone and the benefit of the drug [are] almost exact" and predicted that athletes would be the earliest adopters.

Though it may be years before doctors are writing prescriptions that turbocharge your training, serious people are aimed at that goal. Evans's group is a front-runner in the race, but there are others: independent teams around the world developing naturally derived and synthetically engineered compounds that in preliminary animal experiments—and a few human tests—have measurably increased overall fitness.

Obviously, there will be hurdles. One is convincing biotech firms to back the costly studies required to create a marketable drug. Another is the U.S. Food and Drug Administration, which won't green-light a new treatment that exists solely to help people run farther. (Scientists would first have to show that the drug can cure a real disease.) Even so, Evans believes that we're heading toward an inevitable day in which a pill will supplement and, in many cases, entirely replace exercise.