Abstract: Naphthylmethyl isoquinoline (NI),a modified structure of tetrandrine, was synthesized by the China Pharmaceutical University.The effects of NI on blood pressure of animals and its possible mechanism were studied, in anesthetized rats and cats, iv NI lowered the mean arterial pressure. Meanwhile, the total peripheral vascular resistance was reduced markedly, but the heart rate, cardiac output and cardiac index showed no noticeable change. In rats its hypotensive effect was stronger than tetrandrine when equal toxic doses were compared. In conscious normotensive rats, renal hypertensive rats and SHR, oral administration of this compound resulted in a marked fall of systolic blood pressure which remained at a steady low level. It decreased the pressor response of adrenaline in rats. Intra-arterial injection of NI into the perfusing system decreased the posterior limb and renal Vascular resistance immediately in constant-rate perfusion of the posterior limb and renal blood vessel of rats. It shifted the cumulative dose-response curves of noradrenaline, KCl and CaCl₂ to the right in isolated rabbit aortic rings and depressed their maximal responses with IC₅₀ 9.7,7.2 and 1.6 μmol/L , respectively, which were 2.9, 9.1 and 1.8 times stronger than that of tetrandrine, correspondingly. These results suggest that the hypotensive effect of naphthylmethyl isoquinoline is mainly attributable to its arteriolar dilatation by inhibiting calcium influx.