Both natural and synthetic oxytocin were administered intravenously to un-anesthetized and anesthetized subjects. When given in dilute infusion they produced no cardiovascular effects, but when administered in concentrated doses they produced hypotension and electrocardiographic changes. The electrocardiographic changes were more marked with natural, than with synthetic oxytocin. Laboratory studies on the rat mesoappendix indicated that natural oxytocin induced a transient epinephrine-potentiating effect that was not observed with comparable doses of synthetic oxytocin. Synthetic oxytocin is a safe drug when administered in dilute infusion, and it is compatible with the anesthetic agents commonly used in obstetrics. Natural oxytocin, however, has the disadvantage of containing 2 contaminants: vasopressin and animal protein. The presence of stress may play an important role in the production of cardiac arrhythmias during the administration of intravenous natural oxytocin.