Objective To investigate the vascular effect of hydroxyl-safflor yellow A(HSYA) on rat thoracic aorta and its underlying mechanism. Methods The tension of isolated thoracic aorta rings of rats perfused with different concentrations of HSYA(1?10-6-1?10-4 mol/L) was measured using organ bath technique.The effects of HSYA on the vasocontraction induced by cumulative phenylephrine(PE)(1?10-6-1?10-4 mol/L),KCl(6?10-2 mol/L) and CaCl2(1?10-5-3?10-3 mol/L) were recorded respectively. Results HSYAcaused a concentration-dependent anti-contraction effects by KCl or PE in endothelium-intact and endothelium-denuded aortic rings.HSYA inhibited the CaCl2-induced contraction and downward shifted concentration-response curve of aortic rings.HSYA+HP resulted in more significant anti-contraction effect than single use of HSYA(P0.05).There were significant differences in anti-contraction effect between HSYA+RR and RR or HSYA(P