Unanesthetized dogs were infused with heterologous (hog) renin at 0.33 Goldblatt U/kg per h for 2 h, once normally hydrated and once after 48 h of dehydration. Dehydration increased the average plasma osmolality from 306 to 322 mosmol/kg, the plasma renin activity (PRA) from 0.5 to 1.4 ng/ml per h, and the plasma antidiuretic hormone (ADH) concentration from 1.7 to 3.7 muU/ml, although the latter was not statistically significant. Renin infusion resulted in approximately the same average PRA, about 10 ng/ml per h, in both states of hydration. Mean arterial blood pressure increased during renin infusion in both states of hydration, although the increase was greater when the dogs were normally hydrated. There was no apparent effect of renin infusion on plasma ADH concentration when the dogs were normally hydrated, but in the dehydrated state renin infusion was accompanied by an increase from 3.7 to 6.3 muU/ml in plasma ADH concentrations after 80 min of infusion. There were no apparent changes in plasma osmolality or sodium or potassium concentrations due to the renin infusions; however, plasma osmolality and potassium concentration decreased during the course of the experiment. The results suggest a possible role for the renin-angiotensin system of renin released by the kidney in the control of ADH during dehydration. The metabolic clearance rate of the hog renin was 37 ml/min-kg.