Direct measurements reveal that hypothalamic temperature is significantly higher than that of the tympanic membrane of the cat under Sernyl anesthesia. The posterior hypothalamus is warmer than the anterior hypothalamus. Bilateral carotid occlusion results in prompt elevation in both anterior and posterior hypothalamic temperatures while the tympanic membrane simultaneously cools. Thus, the hypothalamus is normally cooled by its arterial blood supply while the tympanic membrane is warmed by it. The marked differences in relative temperatures, together with opposite temperature changes imposed by arterial occlusion, underline the danger of assuming identity in temperature of these areas under dynamic conditions. The elicitation of sweating in man by a strong heat stimulus applied to occluded extremities strongly supports the hypothesis that cutaneous thermal receptors participate. The merging of a hot sensation into one of severe pain strongly suggests the participation of the C-fiber system of cutaneous afferents in the response. It is concluded that sweating may be elicited by 1) a primary stimulus arising within the thermal receptors of the anterior hypothalamus, 2) stimulation of cutaneous thermal receptors, or 3) some combination of the two. Submitted on June 22, 1962