(The New York Times News Service) -- Dr. Richard Friedman, a psychopharmacologist at Weill Medical College of Cornell University, had a runner's high - once.

"When I was young and foolish, I ran a marathon in the Smoky Mountains," he said. "I have never before or since had that kind of high, and maybe it was just the result of a near-death experience."

Bill Fox, a recreational bicyclist and an IBM research lab technician, says his exercise highs do not come easily.

"You've really got to work for this high," Fox explained, saying he usually needed two hours or more of sweaty, intense, vigorous exercise. But when the feeling comes, he said, it is just like cocaine, a drug he knows from his days as an addict. "It has that well-being kind of feeling, that Superman kind of feeling," said Fox, of Middletown, N.Y.

Others say they never get such a feeling, no matter how hard or how much they exercise. In fact, despite a widespread belief in the so-called runner's high, a feeling of intense euphoria that is supposed to come with vigorous exercise, the experience is not consistent or predictable. Some researchers have asked whether it exists at all.

Many say its supposed genesis, a rush of endorphins into the brain, is without scientific evidence and some say the whole endorphin-runner's-high hypothesis is the scientific version of an urban legend.

"I believe this endorphin-in-runners is a total fantasy in the pop culture," said Dr. Huda Akil, an endorphin researcher at the University of Michigan, who is president-elect of the Society for Neuroscience.

Yet only now are a few researchers rigorously examining exercise as an addictive behavior. They are finding that exercise, in rats at least, may actually be addictive but that it is not at all clear that the crucial brain chemical is an endorphin.

The hypothesis that there is a runner's high and that it is caused by endorphins emerged in the 1970s, when scientists found a new class of brain chemicals that act just like morphine.

"We had meetings to decide what to call it," said Dr. Solomon Snyder, a neurobiologist at Johns Hopkins University, with a committee deciding on "endorphin." The word is an amalgam of endogenous, meaning made by the body, and morphine.

Scientists eagerly looked for evidence that the chemicals mediated any and all sorts of pleasure. Meanwhile, Snyder said, a running craze had begun, with enthusiasts exalting in the wonderful feelings they said exercise elicited.

"Jogging was first becoming popular in the United States, and everyone was talking about the runner's high," Snyder said. "They said: "Well, I have a high. It must be endorphins."'

Indeed, scientists noted that endorphin levels in the blood rose with exercise. But that finding is meaningless, they say. "Endorphins in the blood are irrelevant," Snyder said. They do not pass from the circulating blood into the brain.

It is easy to see how the confusion arose, Akil said, because endorphins are released into the blood during running as part of the body's normal stress response. Scientists still do not understand the function of endorphins in the blood, Akil said, but many erroneously assume that if their levels rise in the blood, they rise in the brain, too.

Scientists could find brain endorphin levels by doing spinal taps on volunteers as they ran. But that, Akil said, "is hardly the noninvasive procedure that might allow one to capture the elation of running," she said. Nor is another method, PET scans of the brain, as people run.

Copyright 2002 The New York Times News Service. All rights reserved.