Biologic actions attributed to adrenomedullin include reduction of arterial pressure and suppression of aldosterone secretion. To assess possible in vivo antiangiotensin II actions of adrenomedullin, we examined hemodynamic and adrenal responses to stepped angiotensin II infusions with or without co-infusions of adrenomedullin (33 ng/kg/min) in conscious sheep under controlled conditions of a low sodium intake. Plasma adrenomedullin levels rose during peptide infusions (p < 0.001) to plateau at approximately 15-18 pM. The dose-dependent pressor response (15-20 mm Hg) of angiotensin II was both delayed and markedly attenuated (p = 0.017) by adrenomedullin, which also stimulated heart rate (p < 0.001) and cardiac output (p < 0.001). Adrenomedullin prevented the angiotensin II-induced increase in peripheral resistance (p < 0.001). Plasma aldosterone responses to angiotensin II were variable and were not significantly altered by concomitant adrenomedullin infusion. In conclusion, low-dose infusion of adrenomedullin administered to conscious sheep on a low-salt diet clearly antagonized the vasopressor actions of administered angiotensin II while stimulating cardiac output and heart rate. The data suggest a possible role for adrenomedullin in cardiovascular homeostasis in part through antagonism of the vasopressor action of angiotensin II.