A generalized knowledge of opiates has been available for centuries, but it was not until 1975, following the isolation of the first endogenous opioids, that it was appreciated that their analgesic properties were mediated via specific receptors. Three general classes of opioids are now recognized: the endorphins, the enkephalins, and the dynorphins. Assaying of the endogenous opioid peptides has never been simple, and now radioimmunoassay has replaced the earlier bioassays. When coupled with immunohistochemical techniques, radioimmunoassay allows precise localization of opioids in various tissue sections. There is evidence that plasma concentration of endorphins, and possibly also met-enkephalin, increases with exercise. In addition, the opioid peptides have important implications for the effects of other hormonal functions, including the pituitary hormones and catecholamines. Few studies during exercise are available, and naloxone has been used widely as the agent to determine the role of endogenous opioids on hormonal secretions. Slightly conflicting results have been obtained, depending on the dosage of naloxone used. With high-dose naloxone, there appears to be overall endorphin inhibition of cortisol, prolactin, and growth hormone secretion. Endorphins do not appear to exert a major influence on the cardiovascular response to exercise, but in high-intensity exercise, they appear to partially inhibit the ventilatory response to exercise.