Objective: To investigate the relaxative characteristics of paeoniflorin on thoracic aortic artery in rat and its mechanism. Method: We perfused the isolated rings and observed the response of NA-induced artery contraction to paeoniflorin under the Ca2+-contained and Ca2+-free bath solutions. In the same way the effects of paeoniflorin on the vascular smooth muscle were observed by adding KCl(60 mmol ·L-1)

and the effect on the contraction of the vascular smooth muscle depending on the intrace11ular calcium and extracellular calcium were also observed by adding NA. We also observed the effect of paeoniflorin on the contraction of rings induced by NA in the presence of L-NAME. Result: Peoniflorin relaxed rat aorta rings precontracted by NA in a dose-dependent manner.But had no effect on the aorta’s contraction induced by intracellular calcium and extracellular calcium as well as KCL(60 mmol ·L-1)

the relaxant effect could be attenuated by L-NAME.There was significant change in the group of paeoniflorin(P<0.05). Conclusion: The results indicate that paeoniflorin relaxes vascular smooth muscle in an endothelium-dependent manner. The mechanisms seem to be related with promoting synthesis and release of NO

but not for opening Ca2+ activated K+ channel as well as the inhibition of Ca2+ influx and release of Ca2+ from intracellular stores.