The role of the subfornical organ (SFO), the organum vasculosum of the lamina terminalis (OVLT), and the area postrema (AP) in the regulation of body fluid balance and blood pressure will be investigated by selective electrolytic destruction of each organ in the dog. Serial measurements of changes in body fluid spaces, plasma electrolytes, and blood levels of angiotensin II (AII), vasopressin, ACTH and plasma renin activity will be correlated with the changes in body fluid balance and blood pressure. The effect of selective lesions of each of these three circumventricular organs (CVO) on intraventricular and intravenous AII-induced drinking, vasopressin and ACTH secretion, and blood pressure will be evaluated. In addition, thirst and vasopressin secretion of lesioned subjects in response to hyperosmotic stimuli will be tested. The effect of selective lestions of the SFO, OVLT, and AP on the development of hypertension will be studied in the two-kidney Goldblatt model which is associated with high endogenous levels of AII. These studies will provide information of the role CVOs play in water economy and the regulation of blood pressure.