To examine the relationship between sleep and brain temperature in the rat, the vigilance states, spectral power density of the electroencephalogram (EEG), hypothalamic temperature (T(hy)), and cortical temperature (Tcr) were recorded for 3 days. A 1-day rise of ambient temperature from 23 to 30 degrees C did not affect the percentage of waking, non-rapid eye movement sleep (NREMS), and rapid eye movement sleep (REMS), but increased EEG slow-wave activity in NREMS in the 12-h dark period. T(hy) was invariably higher than Tcr, but at 30 degrees C the difference diminished because of a rise in Tcr. In contrast to Tcr, T(hy) was only slightly increased at 30 degrees C and only during sleep and in the dark period. Although the temperatures changed largely in parallel at vigilance state transitions, Tcr rose more rapidly than T(hy) at NREMS-REMS transitions and more slowly at NREMS-waking transitions. T(hy) declined more rapidly than Tcr at waking-NREMS transitions and more slowly at REMS-NREMS transitions. The results are consistent with a central role of the hypothalamus in the activation and deactivation of the waking state.