1 In cats anaesthetized with chloralose, the central excitatory substances, tubocurarine, picrotoxin, bicuculline, leptazol and strychnine, were applied to the exposed ventral surface of the brain stem through paired Perspex rings placed across the medulla and their effects on vasopressin release and arterial blood pressure were examined.2 The excitatory substances released large amounts of vasopressin when applied to an area 6-9 mm caudal to the trapezoid bodies. From this area vasopressin release was previously obtained with nicotine.3 With nicotine, the vasopressin release occurred almost instantaneously and tachyphylaxis developed rapidly. With the excitatory substances the release increased gradually and there was no tachyphylaxis. When these substances were applied for several minutes, the release reached its maximum a considerable time after their removal, except with leptazol when release diminished at once after removal.4 The excitatory substances had little or no effect on arterial blood pressure when applied to the vasopressin releasing area, but produced strong pressor responses when applied to a more rostrally situated area.5 It is concluded that the excitatory substances release vasopressin and raise arterial blood pressure because they are antagonists of gamma-aminobutyric acid and/or glycine and that numerous inhibitory neurones which release these amino-acids synapse at the ventral surface of the medulla. The physiological function of those which synapse at the vasopressin releasing area may be to act as a brake on vasopressin release, and of those which synapse at the more rostrally situated area to act as a brake on arterial blood pressure.