Vasopressin antagonist, which antagonizes pressor effect of vasopressin, was injected to spontaneously hypertensive rats under various conditions to observe whether arterial pressure was decreased to indicate secretion of pressor amounts of vasopressin. Vasopressin was secreted in pressor amounts in spontaneously hypertensive rats after acute spinal transection or sinoaortic denervation. This is in sharp contrast to normal rats in which ganglion blockade with hexamethonium bromide is necessary in addition to spinal transection or sinoaortic denervation for secretion of pressor amounts of vasopressin. This indicates that vasopressin secretion in pressor amounts is faciliated in spontaneously hypertensive rats.