To test the hypothesis that angiotensin II releases antidiuretic hormone (ADH) after injection into ventricular cerebrospinal fluid, conscious adult male Sprague-Dawley rats with a lateral cerebroventricular cannula received an intraventricular injection of 0, 10, 50, or 100 ng angiotensin II. Trunk blood was collected 90 seconds later for radioimmunoassay of ADH. Plasma ADH, pg/ml (mean plus or minus S. E.), for the four dose levels were 2.8 plus or minus 0.7, 9.6 plus or minus 2.5, 22.6 plus or minus 5.6 and 25.0 plus or minus 5.0, respectively. The increases produced by angiotensin were statistically significant (p smaller than 0.05). Plasma ADH of the 10 ng group was intermediate between control and the two highest angiotensin doses (p smaller than 0.05), suggesting a dose-response relationship. These data provide direct evidence that angiotensin releases ADH by central mechanisms.