The role of prostacyclin (PGI₂) in human parturition was studied by measuring the concentrations of its main metabolite, 6‐keto‐prostaglandin F_lα, before, during and after term labour in the plasma of 9 women and after delivery in umbilical artery and vein plasma. The mean level of 6‐keto‐PGF_1α rose significantly over the course of labour. The mean level was 266 pg/ml before labour, 341 pg/ml in the presence of regular contraction, 387 pg/ml at a cervical dilatation of 5 cm, 386 pg/ml at delivery, and 397 pg/ml at the expulsion of the placenta. The 6‐keto‐PGF_1α concentration dropped to below pre‐labour levels of 233 pg/ml within 2 hours of delivery, and this level was maintained for the first four postpartum days. The mean levels of 6‐keto‐PGF_1α in the umbilical artery and vein plasma, 1667 and 1193 pg/ml respectively, were significantly higher than the maternal plasma levels, but there was no significant difference between the artery and vein levels. PGI₂ is characterized by its antiaggregatory and vasodilating abilities, so that the rise in PGI₂ during labour may play a role in the prevention of intrapartum thromboembolic complications in the maternal and the uteroplacental circulation.