In rats, brain temperature (Tbr) and core temperature (Tc) were recorded in parallel with the sleep-wake activity throughout the 24-h diurnal cycle, consisting of a 12-h light (L) and a 12-h dark (D) period. In order to characterize the temperature changes associated with the arousal states in the L and the D separately, (i) the average temperatures in wakefulness (W), non-rapid eye movement sleep (NREMS) and REM sleep (REMS), and at the transitions between the arousal states were calculated; (ii) the courses of temperatures before and after the transitions (falling asleep, awakening from NREMS or REMS, transition from NREMS to REMS) were determined; (iii) the rates of changes in Tbr and Tc were calculated for each state; and (iv) the correlations between the temperatures and the overall length of each arousal state, and between Tbr and Tc were studied. In both the L and D periods, Tbr and Tc decreased at the beginning of NREMS, then levelled off, and increased slightly before awakening. Apart from short arousals which did not affect temperature, Tbr and Tc increased in W, peaked 15-20 min after awakening, and declined significantly before the falling asleep. In REMS, Tbr increased at a high rate, while a slight increase in Tc was evident in the L only. Correlations between the temperatures and the arousal states were found in both the L and the D. The courses of Tbr and Tc were also correlated. The results support the existence of characteristic changes in body temperature related to the arousal states in the rat.