Using recent advances in brain physiological, neurohistochemical, and molecular biological techniques, it could be demonstrated that the central action of vasopressin (VP) is important in cardiovascular regulation and in the pathogenesis of hypertension. VP is now known to be located in the area of the brain involved in cardiovascular regulation. Furthermore, in various pathophysiological states, brain VP secretion is regulated separately from the peripheral VP secretion system. The role of brain VP in the regulation of the circadian rhythm of blood pressure is becoming a topic of major interest.