The purpose of this study is to determine the effect of Semen Cuscutae on phenylephrine(PE) and KCl induced contraction of isolated rat thoracic aorta. Contractile force was measured with force displacement transducer under 1.5 g loading tension. Contraction evoked by PE (ED50) was relaxed significantly by Semen Cuscutae. The mean percent relaxing of PE induced contraction was 60.6% (p<0.01) after 300 ㎕/㎖ Semen Cuscutae, but the mean percent contraction of KCl induced contraction was 10.4% (p<0.05) after 300 ㎕/㎖ Semen Cuscutae. Vasorelaxation by Semen Cuscutae was significantly reduced after with and without endothelial cells. Pretreatment of aortic rings with L-NAME (100 μM), a nitric oxide synthase inhibitor, indomethacin (3 μM), an inhibitor of cyclooxygenase, methylene blue (10 μM), guanylyl cyclase inhibitor, propranolol (1 μM), a β-adrenoceptor antagonist, atropine (1 μM), a muscarinic receptor antagonist and diltiazem (10 μM), a Ca2+-channel blocker was recovered the effect aorta. These results indicate that Semen Cuscutae can relax EP induced contraction of isolated rat thoracic aorta and that this increasing contraction related to epithelium and various effect