The effects of intraventricular administration of beta-endorphin on thermoregulatory responses of unanesthetized rats to different ambient temperatures (Ta) of 8, 22 and 30 degrees C were assessed. Administration of beta-endorphin produced a fall in rectal temperature at both Ta 8 and 22 degrees C. The hypothermia in response to beta-endorphin was brought about by both cutaneous vasodilation (as indicated by an increase in both the tail and the foot skin temperatures) and decreases in metabolic heat production. However, at Ta 30 degrees C, administration of beta-endorphin produced no change in rectal temperature or other thermoregulatory responses. Furthermore, the hypothermic effect induced by beta-endorphin was greatly attenuated by either the depletion of brain serotonin levels (with 5,6-dihydroxytryptamine and p-chlorophenylanine) or the blockade of opiate receptors (with naloxone). The data indicate that beta-endorphin leads to hypothermia in rats by increasing sensible heat loss and decreasing metabolic heat production, probably via the release of endogenous serotonin within brain.