A prolonged and frequently extreme pressor response subsequent to the usual pressor response to asphyxia, nicotine injection or carotid occlusion has been observed in dogs, cats and monkeys treated with certain pressor aminies. Analysis of this phenomenon indicates that it is due to sensitization of the vascular bed, largely that of the intestine, by the amines to a pituitary hormone released by the various stimuli used. This hormone is probably equivalent to vasopressin.