Thirty-two pregnant ewes (Saudi breeds) selected during 1998 and 1999, were studied throughout the whole gestational length and around parturition. Concentrations (mean±S.E.) of plasma progesterone (3·9±0·28 ng/ml), oestradiol-17β (20·5±2·33 pg/ml) and cortisol (4·8±0·58 ng/ml) remained fairly constant throughout the early stages of pregnancy. Characteristic changes in these steroids were evident during the prepartal period involving a steady decrease in plasma progesterone beginning 8 days before parturition; a remarkably consistent increase in oestradiol-17β 4–6 days prior to delivery, peaking in excess of 250 pg/ml at the birth day; and a fivefold increase to 23·7±2·12 ng/ml in plasma cortisol 2 days preceding delivery, showing a large increase during the day of lambing. The timing of the marked reduction in the platelet count and the coagulation screening tests including prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT), during the prepartal period, were significantly correlated with the temporal changes in the circulating steroid hormone concentrations. The results suggest that the characteristic prepartal hormonal changes might provide the basis for shifting the fairly balanced coagulable state throughout gestational periods to a mild to moderate degree of hypercoagulability during parturition which is needed to limit the possible intra- and post-partum uterine haemorrhage.