The effect of a rapid increase in circulating melatonin on body temperatures and sleepiness was investigated in eight young adults at 1000. Melatonin administered intravenously at 10- and 30-μg doses, but not 3 μg, resulted in elevated plasma and saliva levels consistent with endogenous levels measured in adults at night. Melatonin at 10 and 30 μg significantly attenuated the daytime increase in rectal core temperature ( P < 0.05 for both). The mean maximum rectal core temperature differences between saline and melatonin treatment were 0.11 ± 0.03°C, 0.16 ± 0.04°C, and 0.18 ± 0.04°C after the 3-, 10-, and 30-μg melatonin doses, respectively. All three doses significantly increased hand temperature compared with saline ( P < 0.05) within 30 min. The mean maximum hand temperature differences were 0.72 ± 0.12°C (3 μg), 0.95 ± 0.15°C (10 μg), and 0.65 ± 0.11°C (30 μg). Foot temperature and subjective sleepiness measures did not change at any melatonin dose. The results suggest that daytime intravenous injection of melatonin to achieve normal nocturnal levels in young adults may produce significant thermoregulatory changes without soporific effects.