Vasomotor effects of serotonin (5-hydroxytryptamine; 5-HT) were studied in isolated human placental chorionic veins in vitro. 5-HT produced marked contraction with a maximum of 89 ± 5.05% of the K+-induced contractions. This response was specifically and competitively antagonized by ketanserin at doses of 10––9 and 10––8M(pA2 = 9.65). Ketanserin, at concentrations up to 10––7M, induced noncompetitive inhibition of the contractions. These findings show a marked vasoconstrictor effect of 5-HT on the capacitance vessels of the human placenta, which suggests that this amine could play an important role as a mediator of fetal circulation. The possible therapeutic effect of ketanserin as a vasodilator to improve blood flow in the uteroplacental unit, is also discussed, based on the ability of this drug to block competitively 5-HT effects on chorionic veins.