Objective : To examine the inhibition of 5-Hydroxytryptamine(5-HT) induced vasocontraction of the water extract of Acanthopanax senticosus Harms roots (ASR) on rat thoracic Aorta and mesenteric artery with and without endothelium. Methods : Segments of thoracic aorta and mesenteric artery obtained from rats immediately after delivery were mounted in organ baths superfused on a polygraph. Results : We found that the thoracic aorta segments responded to the water extract of ASR with a dose-dependent and concentration-dependent vasorelaxation. 5-HT produced a concentration-dependent contraction of the thoracic aorta and mesenteric artery. At high concentrations of ASR, the inhibition responses were 93.7% (Jang-su), 93.5% (Heok-ryong-kang-sung), 92.8% (Mt. Back-doo), and 83.5% (Yeon-byun) of the maximum 5-HT induced contraction. At high concentrations of ASR, the relaxational response at thoracic aorta and mesenteric artery wi1h endothelium were 95.2% and 94.6%; without endothelium were 93.5% and 92.5% of 1he maximum 5-HT induced contraction. Conclusions : In conclusion, the effect of water extract of ASR had potent inhibition at 5-HT and the effect of ASR in isolated thoracic aorta and mesenteric artery showed dose-dependent inhibition but endothelium-independent response.