We used miniature data loggers to record temperature and activity in free-ranging springbok (Antidorcas marsupialis) naturally exposed to severe nocturnal cold and moderate diurnal heat. The animals were active throughout the day and night, with short rests; the intensity of activity increased during daylight. Arterial blood temperature, averaged over many days, exhibited a circadian rhythm with amplitude < 1 degree C, but with a wide range which resulted from sporadic rapid deviations of body temperature. Peak blood temperature occurred after sunset. Environmental thermal loads had no detectable effect on blood temperature, even though globe temperature varied by > 10 degrees from day to day and > 20 degrees C within a day. Brain temperature increased approximately linearly with blood temperature but with a slope < 1, so that selective brain cooling tended to be activated at high body temperature, but without a precise threshold for the onset of brain cooling. Low activity attenuated selective brain cooling and high activity abolished it, even at high brain temperature. Our results support the concept that selective brain cooling serves to modulate thermoregulation rather than to protect the brain against heat injury.