The inhibitory effects of ketanserin and spiroperidol (a neuroleptic drug) on the contractile response of isolated rat uterus to serotonin (5-HT) were investigated. Ketanserin caused non-competitive inhibition of the contractile response to 5-HT and showed more selective inhibition than the other 5-HT antagonists tested. The inhibitory effect of spiroperidol was comparable with the effects of classical 5-HT antagonists. These results suggest that ketanserin and spiroperidol selectively inhibit the contractile response of isolated rat uterus to 5-HT.