In order to investigate the role of endothelin (ET) in blood pressure re-gulation, the depressor effect of ET_A receptor antagonist -BQ_(123) was observed. After in-travenous administration of BQ_(123) (0.1, 1.0 and 2.0 mg/kg) into spontaneously hyperten-sive rat(SHR) and normotensive WKY rat, their blood pressure decreased in a dose-depen-dent manner, which continued 40-60 min. The blood pressure reducing action of BQ_(123)in SHR was more potent than that in WKY rat. Intravenous injection of BQ_(123) intoWistar rat inhibited pressor reponse induced by exogenous ET-1(2?10~(-9)mol/kg) in adose-dependence manner. ET-1(10~(-7)mol/L) stimulated proliferation of cultured smoothmuscle cells from SHR aorta, i.e., increase in ~3H-thymidine and ~3H-Leucine, however, BQ_(123)effectively antagonized the above action of ET-1. These results suggested that endogenousET to some extent could be involved in the regulation of normotension and also in thedevelopment of hypertension, and that ET antagonists would be useful for clinical preven-tion and therapy of hypertensive diseases.