An investigation was made of the effect of angiotensin II on the release of ADH in response to an osmotic stimulus. Dogs anesthetized with pentobarbital were hydrated initially by the infusion of 4% glucose iv, in order to bring the plasma concentrations of ADH and renin to low levels. Subsequent infusion of 2.5M NaCl iv, 1.38 ml/kg per min, resulted in a progressive increase in the plasma ADH concentration in response to the increasing plasma osmolality. The relationship between the plasma ADH concentration and plasma osmolality appeared to be rectilinear over the range studied. When the hypertonic saline infusion was accompanied by an infusion of angiotensin II, 10 ng/kg per min, into a common carotid artery, the increase in the plasma ADH concentration was significantly greater. Infusion of angiotensin II alone in the dose tested and under the reported experimental conditions was without a significant effect on the plasma ADH level. It is concluded that angiotensin II can potentiate the release of ADH in response to an osmotic stimulus. (Endocrinology93: 42, 1973)