In vitro experiments using isolated rings and perfused segments of humanchorionic veins, were designed to determine the effects of serotonin and their interaction with PGF2α on vascular tension and perfusion pressure. These vessels responded vigorously with high sensitivity to the vasoconstrictor effects of serotonin in both experimental designs. Administration of PGF2α also led to constriction of the veins, but this autacoid is at least 100 times less potent than serotonin. When adding PGF2α previously to serotonin to the bathing fluid or to the perfusate, the serotonin response was significantly potentiated, increasing the maximal contractile response in 116 ± 12 % and the perfusion pressure in 180 ± 25%. Both autacoids probably operate in this vascular bed interacting with each other and playing an important role in the regulation of placental blood flow in vivo