1. Changes in temperature were determined following injection of noradrenaline, adrenaline, isoprenaline, dopamine and 5-hydroxytryptamine (5-HT) into the cerebral ventricles of the conscious mouse.2. Noradrenaline (1-20 mug) and dopamine (10-160 mug) caused falls in body temperature. Adrenaline (1-20 mug) caused a slight and transient rise in body temperature followed by a fall. Isoprenaline (5-20 mug) caused a rise in body temperature, hypothermia only occurring after very high doses (200 mug) of this catecholamine.3. alpha- and beta-adrenergic blocking agents, phentolamine (> 2 mug) and propranolol (> 5 mug) respectively, caused falls in body temperature when injected into the cerebral ventricles of the mouse.4. Specific drug antagonism studies were limited owing to the intrinsic effects of the alpha- and beta-adrenergic blocking agents. However, some evidence was obtained to indicate that noradrenaline mediated its effects through a central alpha-type adrenergic receptor.5. 5-HT (10-160 mug) caused a fall in body temperature. The action of this indoleamine and the catecholamines in regard to thermoregulatory function is discussed.