The effects of uterotonic agents (oxytocin and enzaprost) on platelet aggregation in pregnant and nonpregnant women were studied by low-angle light scattering. In nonpregnant women oxytocin produced different effects on ADP-induced platelet aggregation: potentiation at low (<200 nM) and inhibition at high (>400 nM) ADP concentrations. In pregnant women oxytocin did not modulate ADP-induced platelet aggregation or this effect was negligible. Enzaprost competitively inhibited ADP-induced platelet aggregation in all examined women (inhibition constant 84.8+/-25.7 nM).