This study examined whether a 5 mg dose of melatonin induced a lower rectal temperature (Tre) response at rest in both a cool and hot environment while wearing normal military combat clothing, and then examined the influence of this response on tolerance to exercise in the heat while wearing protective clothing. Nine men performed four randomly ordered trials involving 2 h of rest at ambient temperatures of either 23 degrees C or 40 degrees C followed by exercise at an ambient temperature of 40 degrees C. The double-blind ingestion of placebo or melatonin occurred after 30 min of rest. The mean Tre during rest at 23 degrees C had decreased significantly from 36.8 (SD 0.1) degrees C to 36.7 (SD 0.2) degrees C at 90 min following the ingestion of the drug, whereas values during the placebo trial did not change. The lower Tre response during the melatonin trial remained during the first 50 min of exercise in the heat while wearing the protective clothing. Since the final mean Tre at the end of exercise also was significantly reduced for the melatonin [39.0 (SD 0.4) degrees C] compared with the placebo [mean 39.1 (SD 0.3) degrees C] trial, tolerance times approximated 95 min in both conditions. During rest at 40 degrees C, melatonin did not affect the mean Tre response which increased significantly during the last 90 min from 36.9 (SD 0.1) degrees C to 37.3 (SD 0.1) degrees C. This increase in Tre during the rest period prior to donning the protective clothing decreased tolerance time approximately 30 min compared with the trials that had involved rest at 23 degrees C. Total heat storage summated over the rest and exercise periods was not different among the trials at 15 kJ x kg(-1). It was concluded that the small decrease in Tre following the ingestion of 5 mg of melatonin at rest in a cool environment had no influence on subsequent tolerance during uncompensable heat stress.