The aim of the experiments was to study the effects of a moderate heat load on sleep in young (26-day-old) rats and to determine whether the sleep-promoting effect of heat results from stimulation of the homeostatic sleep process. The changes in sleep-wake activity, electroencephalogram slow wave activity (SWA) during non-rapid eye movement sleep (NREMS) and cortical temperature (Tcrt) were determined during and after long (24-h) and short (2.5-h) heat loads (elevation of ambient temperature from 26 degrees C to 32 degrees C), and after total sleep deprivation (SD) combined with a short-term heat load. The heat exposures elicited increases in Tcrt and rectal temperature (2 and 1.7 degrees C respectively). The long-term heat load induced persistent, albeit slight enhancements in NREMS. Rapid eye movement sleep (REMS) increased with a 12-h delay during the 24-h heat load. Heat elicited an immediate large increase in SWA. After this initial increase, SWA declined and tended to fall below the baseline level during the last 12 h of the 24-h heat load. SWA and REMS were significantly suppressed after termination of 24-h heat loading. The increased SWA during the short-term heat load was not followed by subsequent alterations in sleep when the ambient temperature had returned to normal. However, after the combination of SD with the short-term heat load the durations of NREMS and SWA were significantly enhanced compared with those found after SD at 26 degrees C. The results are interpreted as suggesting that heat increases NREMS in the young rat by the same mechanism as is involved in the enhancement of NREMS after SD: a stimulation of sleep drive.