We have studied the serum TSH responses to alterations in body temperature by heating twelve patients with primary hypothyroidism for 48 h and determining the initial and final TSH levels. In four control patients we estimated the TSH responses to rest and exercise. In a group of three patients, the TSH levels were recorded at frequent intervals over a 48 h period, during which they were rested and subsequently warmed. Our results show that in the twelve patients initially investigated, there was a significant fall in TSH. In the smaller group of three patients studied more intensively, TSH levels were inversely related to cutaneous and core temperature. In primary hypothyroidism alterations in body temperature influence the levels of circulating TSH.