Objective: To investigate the vasodilative effect and the possible mechanisms of aqueous extract of Arctium lappa roots on the thoracic aorta rings of rats. Method: The study was performed with the model of isolate rat thoracic aorta rings in organ bath. Result: Aqueous extract of A. lappa roots did not directly relax or constrict vascular rings. Aqueous extract of A. lappa roots (0.075-15 g· L-1) caused concentration-dependent relaxation with Phenylephrine (PE 5×10-6mol· L-1) preconstricted aorta rings with or without endothelium. The extent of relaxation was larger in endothelium-intact aortic rings than that in endothelium-denuded aortic rings. Relaxation effects were reduced partially but significantly by pretreatment with (ω-nirto-L-arginine methyl ester (L-NAME

10-4mol· L-1)

1H-[1

2

4] oxadiazolo [4

3

a] quinoxalin-1-one (ODQ

10-5mol· L-1) in endothelium-intact aortic rings. However

they were not affected by indomethacin(10-5mol· L-1). The relaxation response of aqueous extract of A. lappa roots was significantly enhanced by potassium channel inhibitors Tetraethylammonium chloride(TEA). The aqueous extract of A. lappa roots caused concentration-dependent relaxation in 40 mmol· L-1 KCl aorta rings. Conclusion: Aqueous extract of A. lappa roots causes relaxation of aortic rings through endothelium-dependent and independent pathways. The mechanisms might be involved the release of NO from endothelium and is possibly mediated by the activation of Ca2+ channel.