A single injection of 25 micrograms melatonin induced hyperthermia in rats. The mechanism(s) by which melatonin induces hyperthermia is not clear. In this study the thermotrophic effects of melatonin were studied in adrenalectomized (ADX) and thyroidectomized (TX) rats to ascertain if melatonin-induced hyperthermia is mediated through these glands, as both these glands play a significant role in mammalian thermoregulation. Melatonin (25 micrograms) was administered as a single subcutaneous injection at 06.00, 12.00, 18.00, and 24.00 h. Body temperatures were recorded, using an anal thermometer at hourly intervals, round the clock. In the scotophase the animals were located using dim red light and injected. Although the administration of melatonin to both intact and ADX rats resulted in a rise in body temperature, the effect was more pronounced in the former compared to the latter group. Melatonin induced hyperthermia only during the photophase of the diurnal cycle; in the scotophase the response was either poor or nil. In TX rats melatonin was ineffective in inducing hyperthermia in either the photophase or scotophase. The body temperatures of TX rats and melatonin-injected TX rats were parallel. The results indicate that in ADX rats melatonin induced minor hyperthermia, whereas in TX animals it failed to produce any hyperthermia. The findings suggest that melatonin induces hyperthermia probably through its action on thyroid activity. Other mechanisms of action cannot be ruled out, however.