Simultaneous temperature measurements in the hypothalamus and arterial blood reveal that the thermosensitive elements in the hypothalamus are normally cooled by the blood perfusing it. Posterior hypothalamic temperatures are consistently higher than those in the anterior hypothalamus. Occlusion of the common carotid arteries elicited a marked elevation in hypothalamic temperatures. Small i.v. doses (2.5 to 10 mg/kg) of sodium pentobarbital resulted in a prompt and sustained decline. The fall in temperature after pentobarbital appears to be due to a significant increase in hypothalamic blood flow.