The blood pressure and heart rate effects of the specific pressor antagonist of vasopressin d(CH2)5Tyr(Me)AVP, 5 micrograms i.v. was evaluated in conscious normotensive rats. Our results suggest that a baroreceptor reflex mediated decrease in sympathetic nerve activity, as reflected by a slowing in heart rate, returns blood pressure to baseline levels shortly after injections of exogenous vasopressin. This has to be taken into account when interpreting the hemodynamic response to vasopressin antagonists in rats pretreated with this vasopressive hormone.