In order to investigate the physiological role of the brain renin-angiotensin system in the regulation of vasopressin (ADH) release, angiotensin II (Ang II, 10 ng/kg/min) or l-Sar-8-Ile-Ang II (50 ng/kg/min), an Ang II antagonist, was administered intracerebroventricularly to dogs (n = 42) anesthetized with urethane and chloralose after morphine sedation. The effects of the intravenous infusion of either 0.15 M or 2.5 MNaCl (0.1 ml/kg/min, 75 min) were also studied. In control dogs, artificial cerebrospinal fluid (ACSF) was administered at a rate of 10 µl/min for 105 min. ACSF given intracerebroventricularly plus 0.15 M NaCl given intravenously did not affect ADH release, but 2.5 MNaCl given intravenously raised the plasma ADH level in parallel with the rise in plasma osmolality. Heart rate and blood pressure did not change significantly in ACSF along with 0.15 M NaCl, but heart rate increased significantly in ACSF along with 2.5 M NaCl. Ang II along with 0.15 M NaCl significantly raised plasma ADH and decreased heart rate without any changes in blood pressure. Ang II along with 2.5 M NaCl brought about a significant rise in plasma ADH level, arterial blood pressure, heart rate, and plasma osmolality. But simultaneous application of Ang II and 2.5 M NaCl did not result in a larger rise in plasma ADH than that expected from the effects of the two stimulations given separately. Namely, Ang II did not potentiate ADH release elicited by osmotic stimulation. Ang II antagonist given intracerebroventricularly neither affected ADH release and the cardiovascular system in 0.15 M NaCl nor inhibited ADH release in response to osmotic stimulation. Ang II antagonist completely blocked the effects of Ang II. These results clearly show that Ang II given intracerebroventricularly stimulates the receptors within the brain to elicit ADH release, but does not potentiate ADH release provoked by peripheral osmotic stimulation. Ang II does not stimulate the cardiovascular system during 0.15 M NaCl challenge, but stimulates it during 2.5 MNaCl challenge