M-1 is a major metabolite of sarpogrelate, a selective inhibitor of 5-hydroxytryptamine (5-HT) _2A receptor. Our aim was to evaluate the effect of M-1 on the 5-HT-induced vasoconstriction in isolated endothelium denuded human internal thoracic artery (ITA) obtained from patients undergoing coronary bypass surgery. We investigated the effects of M-1, sarpogrelate and SB224289, a selective antagonist of the 5-HT_1B receptor, on the 5-HT-induced vasoconstriction. The vasoconstriction induced by 5-HT was significantly inhibited by M-1 in a concentration-dependent manner. Supramaximum concentrations of sarpogrelate or SB224289 significantly, but not completely, inhibited the 5-HT-induced vasoconstriction. In addition, simultaneous pretreatment with both sarpogrelate and SB224289 almost completely inhibited the 5-HT-induced vasoconstriction. M-1 also significantly almost completely inhibited the 5-HT-induced vasoconstriction, which mimics the effects of the simultaneous pretreatment with sarpogrelate and SB224289. These results demonstrate that M-1 inhibits 5-HT-induced vasoconstriction via the blocking activity of not only 5-HT_2A receptors but also 5-HT_1B receptors in human ITA.