Reserpine causes a loss of noradrenaline and of 5-hydroxytryptamine from the brain, and many attempts have been made to correlate the clinical effects of reserpine with this loss. One of the difficulties in attempting such a correlation lies in the lack of knowledge about the part played by noradrenaline and 5-hydroxytryptamine in normal brain function. Regions containing the central representation of the sympathetic system (hypothalamus, reticular formation of mid-brain and of medulla) are rich in both these amines. If the stores of the amines have an essential role in central sympathetic activity, one might expect such activity to be impaired after the administration of reserpine. The following observations would appear to support such a possibility: peripheral adrenergic neurones fail to excite the tissues they innervate when reserpine has reduced their stores of amines to 10 % or less (Muscholl & Vogt, 1958); continued sympathetic activity, elicited by drugs which stimulate the sympathetic centres, is accompanied by a reduction in the noradrenaline content of these centres, a fact which demonstrates some connexion between nervous activity and noradrenaline turnover (Vogt, 1954).The object of this work, on which a preliminary note has been published (Iggo & Vogt, 1959), was to test central sympathetic activity in cats in which reserpine administration had reduced the cerebral noradrenaline and 5-hydroxytryptamine to very low figures. Efferent discharge of the preganglionic fibres of the cervical sympathetic was used as a measure of central sympathetic activity. In order to assess possible changes produced by reserpine, it was necessary first to observe the preganglionic sympathetic activity of normal cats in the same experimental conditions, and to devise some simple means of eliciting reproducible changes in activity.