Asp1,Val5-angiotensin II (AII) caused dose-dependent increases in aortic blood pressure (BP) when injected into anesthetized Pekin ducks. Norepinephrine (NE) and epinephrine (E) concentrations in arterial plasma were found to be elevated 1 min after injection of AII, when BP was peaking, while plasma levels of unconjugated dopamine (DA) did not undergo any consistent change. Pretreatment with 6-hydroxydopamine (6-OHDA) rendered ducks supersensitive to the pressor effect of injections of NE, but did not affect adrenal chromaffin cell granulation, plasma NE and E concentrations, or pressor responses to tyramine and AII. Depletion of endogenous NE and E stores, following treatment with reserpine, did not affect pressor responses to NE, but diminished pressor responses to tyramine and AII. The discrepancies in the responses of 6-OHDA-treated ducks and reserpinized ducks to exogenous NE and AII indicate different pressor mechanisms for the two hormones. It is suggested that mobilization of endogenous NE and E contributes to the rise in BP observed following AII injection.