The endogenous opioids seem likely to be assigned a significant role in the integrated hormonal and metabolic response to exercise. This article reviews the present evidence on exercise and the endogenous opioids, and examines their involvement in a number of widely disparate physiological processes. In considering the role of individual opioid peptides, it is important to remember that many of the tools and techniques now used are still relatively crude. Most studies have demonstrated that serum concentrations of endogenous opioids, in particular beta-endorphin and beta-lipotrophin, increase in response to both acute exercise and training programmes. Elevated serum beta-endorphin concentrations induced by exercise have been linked to several psychological and physiological changes, including mood state changes and 'exercise-induced euphoria', altered pain perception, menstrual disturbances in female athletes, and the stress responses of numerous hormones (growth hormone, ACTH, prolactin, catecholamines and cortisol). Many reports have described a role for the endorphin response as seen during exercise and have used the opioid receptor antagonist, naloxone, to investigate and verify the degree of involvement of the opioids. However, whether the observed increases in peripheral endorphin concentrations are sufficient to cause immediate mood changes, create menstrual cycle dysfunction or alter pain perception is still not resolved. A relatively new implication for the endorphins and associated changes with exercise is in ventilatory regulation. A number of studies have suggested that endogenous opioids depress ventilation and may, therefore, play a role in ventilatory regulation by carbon dioxide, hypoxia and exercise. It may also be possible that during exercise, the perception of fatigue is modulated by an increase of endogenous opioids.