1 Noradrenaline (0.2 to 20 micrograms) and carbachol (0.1 to 1 microgram) injected into the preoptic/anterior hypothalamic area, evoked dose-dependent falls in core temperature at all sites tested, followed in most experiments by delayed increases that were not dose-related. Muscarine (0.1 to 10 microgram) produced effects similar to those evoked by carbachol. 2 These falls in core temperature were associated with increases in tail temperature, locomotor activity and CO2 elimination (a measure of metabolic rate). 3 The temperature responses to noradrenaline (10 microgram) and to carbachol (1 microgram) were antagonized by intrahypothalamic injections of phentolamine (10 microgram) and atropine (1 microgram), respectively. 4 Analysis of the temperature responses and their respective latencies indicates that carbachol-induced hypothermia was mediated by cholinoceptors in the anterior hypothalamus, whereas hypothermia after noradrenaline was mediated by adrenoceptors throughout the preoptic/anterior hypothalamic area. 5 Vasodilatation of the tail blood vessels contributed significantly to the hypothermia evoked by carbachol, and to that evoked by injections of noradrenaline into the anterior hypothalamus. 6 Hypothermia induced by noradrenaline injection into the preoptic area, was mediated by effector mechanisms additional to non-evaporative heat loss.