Abstract 1. 1. α 1-adrenergic potencies of SM911 and SM2470, whose chemical structures are similar to that of prazosin, a selective α 1-adrenoceptor blocker, were tested in rabbit aortic strips, rat aortic strips and rat vas deferens preparations. 2. 2. SM2470 was as potent as prazosin in α 1-adrenoceptor blocking effects, though SM911 was 0.5-0.1 as potent as prazosin. 3. 3. The p A 2-values for prazosin, SM911 and SM2470 were approximately one order of magnitude lower in rabbit aortic strips and rat vas deferens preparations than in rat aortic strips, suggesting that α 1-adrenoceptors in these tissues may not be identical. 4. 4. SM911 and SM2470 as well as prazosin did not interact with α 2- and β-adrenoceptors, muscarinic and nicotinic cholinoceptors, and histamine and serotonin receptors in doses up to 10 −5 M.