A review of the literature suggests that exercise may have antidepressant effects and, like other treatments for depression such as electroconvulsive therapy (ECT), antidepressant medication, and REM sleep deprivation, may enhance aminergic synaptic transmission in the central nervous system. In addition, the effects of exercise and other antidepressants on sleep are similar. Therefore, it is suggested that exercise is an antidepressant that enhances aminergic synaptic transmission in the central nervous system. Many more psychological and physiological studies must be performed in order to verify and quantify this relationship. Present statements that single out norepinephrine, dopamine, or serotonin as the crucial amine may be premature and oversimplified. Future physiological studies must take into consideration the advantages and disadvantages of human and animal subjects. Future psychological studies should be attentive to possible differences in psychological benefits between normal and depressed subjects and should not neglect the possible role of cognitive factors such as subjects' attitudes towards exercising or the feelings of accomplishment that may result from increased physical fitness. There is also a need to measure antidepressant effects in long-term exercise programs and in studies employing various forms of exercise.