Preganglionic cervical and splanchnic sympathetic activity was recorded before and during administration of inhalation anaesthetics, in rabbits ventilated with oxygen and given gallamine. During control periods, when light anaesthesia was maintained with pentobarbitone, sympathetic discharge responded to changes in arterial pressure. Increased arterial Pco 3 exaggerated the amplitude of the respiratory sympathetic rhythm, and had a more variable effect on the mean impulse discharge rate. Preganglionic activity was increased by 25-50 per cent cyclopropane, which usually raised arterial pressure; by halothane, which caused severe hypotension; and by diethyl ether, which produced smaller circulatory changes. These experiments question the concept of "central vasomotor depression" during inhalation anaesthesia in the rabbit. Changes in preganglionic sympathetic activity produced by inhalation anaesthetics have not been studied directly apart from a brief communication by Martin and Marrazzi (1942), stating that cyclopropane and chloroform do not affect cervical sympathetic discharge in cats. Deutsch, Linde and Price (1962), however, measured an increased plasma adrenaline level during cyclopropane anaesthesia in the dog, while in the same species Millar and Morris (1960) found no increase in circulating catecholamines when halothane was given. The former result suggests that excitation of the sympathetic nervous system by cyclopropane may account for the arterial hypertension which usually occurs. This view is supported by head perfusion experiments (Price et al., 1963), also in the dog, while similar studies suggest that there is depression of central sympathetic discharge during halothane anaesthesia (Price, Linde and Morse, 1963), which accords with the reduced arterial pressure and the catecholamine measurements.