1 In unanaesthetized cats, the administration of 6-hydroxydopamine (6-OHDA), 750 mug, via the lateral cerebral ventricle produced a pronounced fall in rectal temperature but only a slight fall when repeated 7 days later. At this time hypothalamic noradrenaline concentration had diminished to 4% of control.2 In these animals, the hypothermic response to exogenous noradrenaline, 100 mug, given via the same route was uninfluenced by pretreatment with 6-OHDA.3 In unanaesthetized rabbits, intraventricular noradrenaline, 100 mug, produced a rise in rectal temperature and a biphasic effect on arterial pressure, a rise lasting 30 min followed by a fall.4 Intraventricular 6-OHDA, 750 mug, in unanaesthetized rabbits produced a rise in body temperature and a rise in arterial pressure. The same dose given to rabbits depleted of central noradrenaline with central 6-OHDA produced rises in body temperature and arterial pressure of similar magnitude, but of slower onset.5 These results suggest that intraventricular 6-OHDA releases noradrenaline from central neurones and that these neurones subserve thermoregulatory functions in both species. In the rabbit, central noradrenergic neurones can raise arterial pressure.