In the present study, the effects of lycium barbarum polysaccharide (LBP) on endothelial function in the two-kidney, one clip model of hypertension were observed. The results showed that the increase of blood pressure in hypertension rats (HR) could be prevented significantly by treatment with 10% LBP. In isolated aortic rings of LBP-treated rats, the contraction of phenylephrine (PE) was reduced as compared with HR rats. Removal of the endothelium abolished the difference of PE-induced vasoconstriction among groups. In vitro incubation of aortic rings from LBP-treated rats with methyl blue (MB) or N-nitro-L-arginine methyl ester (L-NAME) increased the magnitude of PE-induced contraction. Meanwhile the response to acetylcholine (ACh) was significantly increased in LBP-treated rats, but the response to nitroprusside had no significant difference among groups. Pretreatment with L-arginine partially restored ACh-induced relaxation in RH rats, but no effect in LBP-treated rats. These results suggested that the role of LBP in decreasing vasoconstriction to PE may be mediated by increase of the effects or/and production of endothelium-derived relaxation factor (EDRF). LBP increased formation of EDRF may be related to increase the substrate of EDRF.