Release of arginine vasopressin (AVP) from rat neurohypophysis in in vitro studies is significantly augmented by the addition of angiotensin (A-II), and in in vivo studies in dogs renin and A-II were found to stimulate secretion of AVP. Both these results suggest the existence of a direct relationship between the salt regulating renin-angiotensin-aldosterone system and the water controlling AVP system. To evaluate whether such observations apply also in man a sensitive double antibody radioimmunoassay for AVP was developed [17, 18]. Basal plasma levels of AVP in recumbent humans without salt and fluid restriction at room temperature were 3.4 plus or minus 2.2 pg/ml, and 30 min after the onset of an A-II infusion at a concentration of 3-30 ng/min-kg, a significant increase of AVP was found. Maximum measurements were 2-5 times basal levels which returned to normal within 90 min. During the A-II infusion one also noted a 20 mm Hg rise in blood pressure, accompanied by a significant decrease in plasma renin activity. During the same period serum osmolality and serum sodium concentration did not change. Elevation of blood pressure by norepinephrine was not followed by any detectable change of plasma AVP levels, thus excluding a nonspecific blood pressure effect.