Intracerebroventricular (ICV) injections of arginine vasopressin (AVP) in doses of 0.015 nmoles and 0.15 nmoles produced a fall in mean actual pressure heart rate and respiration in pentobarbital anesthetized rats. The changes in mean arterial pressure and respiration after the higher dose were significantly different from saline injection. In contrast, the same doses of AVP given to conscious animals increased both blood pressure and heart rate. Following the 0.15 nmole dose, there was a marked and significant rise in plasma norepinephrine and epinephrine, indicating that activation of the sympathetic nervous system was, at least in part, responsible for the rise in blood pressure. Plasma vasopressin increased by less than 10 pg/ml following injection. Similar doses of a vasopressin pressor antagonist had no significant effect on mean arterial pressure or heart rate. These results indicate that ICV injection of vasopressin has different effects on blood pressure, depending on the presence or absence of anesthesia: depressor responses in the anesthetized animal and pressor responses in the unanesthetized animal.