Rats with medial preoptic area (MPOA) damage lack adequate autonomic thermoregulatory reflexes but can behaviorally regulate their body temperatures (Tb) in the cold and heat. We examined the performance of such rats on 90-min behavioral thermoregulatory tests. Tests were conducted during the day and night to determine the preferred Tb at different phases of the circadian Tb rhythm. The amplitude of this rhythm was exaggerated after electrolytic MPOA lesions; differences of 3-4 degrees C between daily peaks and troughs were seen (from 37 degrees C during day to 40-41 degrees C at night) compared with control variations of only 1.5-2.0 degrees C (from 36.0-36.5 degrees C during day to 38 degrees C at night). When escaping cold during the day or night or when escaping heat at night, both MPOA-damaged and control rats maintained Tb within +/- 0.5 degrees C of initial values. When escaping heat during the day, control and MPOA-damaged rats allowed their TbS at the end of the 90-min behavioral tests to rise to normal night levels (38 degrees C for controls and 40-41 degrees C for MPOA rats). These results demonstrate that rats with MPOA damage behaviorally defend the different phases of their exaggerated Tb rhythm in a manner similar to that of controls. Therefore the exaggerated swings in Tb seen in these preparations have an active regulated component and are not simply passive results of an excessive rhythm in heat production or heat conservation.