The mechanism(s) responsible for arterial vasodilation observed following acute administration of racemic carvedilol, a novel vasodilator/beta adrenoceptor antagonist, has been investigated in rats. In conscious spontaneously hypertensive rats, carvedilol (0.03-3.0 mg/kg, iv) produced a dose-dependent reduction in blood pressure with no significant effect on heart rate. Because cardiac output was relatively unaffected, the antihypertensive response of carvedilol was associated with a dose-dependent reduction in total peripheral vascular resistance. Submaximal antihypertensive doses of carvedilol were chosen for mechanism of action studies in pithed rats. Carvedilol (0.3 mg/kg, iv) produced a significant inhibition of the beta 1 adrenoceptor mediated positive chronotropic response to isoproterenol. This same dose of carvedilol also inhibited, but to a lesser degree, the beta 2 adrenoceptor mediated vasodepressor response to salbutamol in pithed rats whose blood pressure was elevated by a constant intravenous infusion of angiotensin II. Thus, carvedilol blocks both beta 1 and beta 2 adrenoceptors at antihypertensive doses, with modest selectivity being observed for the beta 1 adrenoceptor subtype. Carvedilol produced significant inhibition of the alpha 1 adrenoceptor mediated pressor response to cirazoline in the pithed rat, but had no effect on the alpha 2 adrenoceptor mediated pressor response to B-HT 933, suggesting that carvedilol is also an alpha 1 adrenoceptor antagonist at antihypertensive doses. Carvedilol had no effect on the pressor response elicited by angiotensin II, indicating a lack of nonspecific vasodilator activity. The vasopressor response to the calcium channel activator, BAY-K-8644, which is mediated through the opening of voltage dependent calcium channels and the subsequent translocation of extracellular calcium, was significantly inhibited by carvedilol (1 mg/kg, iv), suggesting that carvedilol is also a calcium channel antagonist, consistent with our previous in vitro studies. In anesthetized spontaneously hypertensive rats, the antihypertensive activity of carvedilol was nearly abolished by combined pretreatment of the rats with high doses of the alpha 1 adrenoceptor antagonist, prazosin (1 mg/kg, iv), and the nonselective beta adrenoceptor antagonist, propranolol (3 mg/kg, iv), suggesting that the majority of the antihypertensive response produced by carvedilol may be accounted for by blockade of beta and alpha 1 adrenoceptors. We therefore conclude that carvedilol, at antihypertensive doses, is an antagonist of beta 1, beta 2, and alpha 1 adrenoceptors, and also of calcium channels in vascular smooth muscle.(ABSTRACT TRUNCATED AT 400 WORDS)