Prozac and its competitors have spawned best-selling books, racked up sales of more than $10 billion annually and reshaped the clinical treatment of depression.

But an altogether different treatment that shows tremendous promise in alleviating depression has gone largely unnoticed. That treatment is exercise. A growing body of medical literature, including at least three 2005 studies, is showing that aerobic routines as well as weight lifting are effective at combating depression. In addition to the famous "runner's high," or endorphin surge that provides a temporary mood lift following a workout, the studies show that there is a round-the-clock relief that sets in several weeks after the establishment of a regular exercise routine.

A study in the January issue of the Journal of Preventive Medicine suggested that a half-hour a day of exercise six days a week - which is the amount the federal government recommends for all Americans - might be ideal. Comparing two groups of depressed patients, the study found that the group that performed only 80 minutes of exercise a week received little to no mental-health benefit. But the three-hour-a-week group had a substantial reduction in symptoms. The study concluded, "The response and remission rates in the (three-hours-per-week) group are comparable to other depression treatments, such as medication or cognitive behavioral therapy."

Certainly, the need exists for additional tools to fight depression. It is a primary cause of suicide - which is one of the top 15 causes of death - and its burden on patients and on society is second only to that of heart disease, according to a U.S. Surgeon General's report. Between 20 percent and 30 percent of depression patients don't respond to medication after a year of taking it, says Douglas G. Jacobs, associate clinical professor of psychiatry at Harvard Medical School.

Nobody, however, is suggesting that depressed patients need only start running around the block. "I wouldn't say to anybody that exercise should be at the top of their list," says Gerald P. Koocher, president elect of the American Psychological Association. "First, get a good diagnostic evaluation. Then get your mood stabilized." And then add exercise to other treatments. Studies have shown that a combination of antidepression treatments are more effective than a single treatment.

If, as some studies have suggested, exercise is even remotely as effective as medication, then in some respects it could be an ideal treatment. It's less expensive than medication and psychotherapy. It doesn't cause sexual dysfunction or harbor potentially fatal side effects when combined with other drugs, as do some antidepressants. Exercise as a mental-health treatment has dramatically positive side effects: enhanced cardiovascular health and waist-line management.

But there are also obvious problems. Mental-health experts usually aren't fitness trainers and have no way of monitoring patient compliance with a workout regimen. Moreover, does it make sense to prescribe 30 minutes on the treadmill to a patient who can barely climb out of bed? It "is challenging enough for those who are not suffering from depression" to exercise three hours a week, says Harvard's Dr. Jacobs.

Yet depressed patients who are motivated enough to get help in many cases might be motivated enough to exercise on their own. And once they start, studies suggest many will get hooked. In surveys, people who exercise regularly cite as its main appeal its mental and emotional rewards. "The most prevalent reason that people stick with exercise is its effect on mood," says Andrea Dunn, a doctor of behavioral science whose research has focused on exercise.

But is anybody telling depressed patients about exercise? Unlike depression medication - which has behind it the research and marketing clout of the pharmaceutical industry - exercise has behind it little research or advertising money. Consequently, the studies supporting it are typically too small to win publication in major medical journals. Without any marketing push, the studies are getting little attention.

Interviews with psychiatrists - medical doctors whose specialty is mental health - suggest that some aren't even aware of the body of literature supporting exercise as a treatment for depression. When the American Psychiatric Association is asked to provide a psychiatric expert on exercise and depression, it recommends James Lake, a California psychiatrist who says he believes his specialty is too narrowly focused on medication.

"Because of collective professional values and financial interests of academic psychiatry, research priorities have almost exclusively targeted psychopharmacology," says Dr. Lake. However, Dr. Lake points out that the association recently created a committee to research and provide information about alternative therapies, including exercise.

Psychologists are much more aware of exercise as an antidepressant. Indeed, psychologists have produced the majority of the research on exercise and depression. The Web site of the American Psychological Association features an entire page describing exercise as a legitimate third leg of treatment, along with psychotherapy and medication.

Later this month, the annual meeting of the American Psychiatric Association will feature a seminar called, "Does exercise have a place in psychiatry?" One of the two presenters of that seminar isn't a psychiatrist but rather a doctor of psychology, James Annesi.

The other presenter is a 77-year-old psychiatrist who years ago deduced from his own running experience that exercise had tremendous mood-altering potential. The psychiatrist, Sheldon B. Cohen, also devised a way to get his patients exercising: He walked and ran with them during psychotherapy sessions.