Infusion of oxytocin into one vertebral artery of anesthetized dogs did not alter plasma vasopressin concentration, blood pressure or heart rate. However, there was a significant (p < 0.01) increase in plasma renin activity (PRA; Δ = 7.6 ± 2.3 ng/ml · h). A 35% hemorrhage caused blood pressure to fall by 9.4 ± 4.0 mmHg (p < 0.01) and PRA to rise by 8.8 ± 2.7 ng/ml · h (p < 0.05). In 8 dogs that were subjected to a similar hemorrhage and that also received an intravertebral infusion of oxytocin, blood pressure was maintained and PRA increased by 14 + 4.3 ng/ml · h (p < 0.05). Heart rate and plasma vasopressin responses were similar in both hemorrhage groups. The results indicate that oxytocin prevented the fall in blood pressure associated with a hemorrhage, possibly by increasing renin release.