We demonstrated that MCI-9042 potently inhibited the vasoconstrictory effects of serotonin in the isolated perfused hindlegs of the rat. Concentrations of MCI-9042 in the range of 0.01-1 mumol/l caused a dose-dependent inhibition of the vasoconstrictory effect of serotonin (1-30 micrograms/0.1 ml). The maximal inhibitory effect of MCI-9042 was about 97% (1 mumol/l). MCI-9042 caused a concentration-dependent shift to the right of concentration-response curve to serotonin in the rat tail artery. The present data demonstrate MCI-9042 as a potent 5-HT2 receptor antagonist.