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To study the role of hypothalamic thermoreceptors in behavioral thermoregulation, four baboons adapted to living in primate chairs were trained to bar press on a variable interval schedule for peripheral infrared heat. Cooling the preoptic/anterior hypothalamic region for 1.5 hr in neutral Ta (24°C) led baboons to bar press rapidly for heat reinforcement during the initial 30–60 min that body temperature recorded from the midbrain rose 1–2°C. During this time baboons seemed alert, exhibited cutaneous vasoconstriction and often shivered. After the temperature reached a plateau at hyperthermic levels, baboons became quiet, decreased bar pressing for heat, stopped shivering, and peripheral vasoconstriction appeared reduced. Hypothalamic cooling performed in hot (40°C) ambient temperature similarly appeared to lead baboons to work for heat. After intravenous infusion of bacterial pyrogen (Piromen) baboons …