ObjectiveTo survey the vasodilative effect of the extract of huoxuezhuyu recipe and to probe into its underlying mechanisms. MethodsThe vasodilative effect of the drug was studied with the guinea pig mesenteric microvasculature and isolated rat aortic rings. The content of nitric oxide(NO )in the blood plasma and aortic tissue, the activities of nitric oxide synthase (NOS) , constitutive nitric oxide synthase (cNOS) and inducible nitric oxide synthase (iNOS) in the aortic tissue were determined with the method of nitroreductase chromatometry. ResultsThe extract of huoxuezhuyu recipe was shown to counteract the vasocontrictive effect of norepinephrine on the mesenteric microvasculative of the guinea pig. It could also relax the isolated rat aortic rings constricted by phenylephrine. The vasorelaxing effect of the extract persisted despite pretreatment of the aortic rings with glibenclamide or propranolol. However, the extract of huoxuezhuyu exerted vasoconstrictive effect on the aortic rings if the endothelium had been denuded. The extract in high dose was shown to very significantly increase the content of NO in the blood plasma and the content of NO as well as the activities of NOS and cNOS in the aortic tissue. In contrast, the extract in low dose could significantly increase the activity of cNOS in the aortic tissue. ConclusionThe extract of huoxuezhuyu recipe was shown to have a vasodilative effect and its underlying mechanisms may be related to the activation of the vascular NOS and cNOS as well as the promotion of the release of NO from the endothelium of the blood vessel.