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Monatepil, a novel calcium antagonist, has α1-adrenoceptor blocking activity; in the present study, we examined the relative contribution of this α1-blocking activity to its hypotensive effect. Monatepil and diltiazem produced dose-dependent hypotensive effects in anesthetized rats with the same potency. Prazosin and monatepil inhibited the L-phenylephrine (L-PE)-induced pressor response, whereas diltiazem scarcely did. The injection of prazosin produced a decrease in blood pressure (BP) in anesthetized rats. The decrease was recovered with angiotensin II (AH) infusion in a dose-dependent manner. We developed a new rat model by first intravenously injecting prazosin and then infusing All in anesthetized rats. In this model, diltiazem produced the same hypotensive effect as it did in pretreated conditions, although the hypotensive effect of monatepil was attenuated by 20–35% as compared with pretreated conditions. These results suggest that monatepil exerts α1-adrenoceptor blocking action in vivo and 20–35% of the hypotensive effect of monatepil is attributed to its α1-adrenoceptor blocking activity.