Initiation of parturition in the rabbit by maternal and foetal administration of cortisol: effect of rate and duration of administration: suppression of delivery by progesterone

Continuous infusion of cortisol at rates from 0.31 to 1.24 mg/h into pregnant rabbits from day 21 of pregnancy onwards induced delivery in approximately 72 h. Similar rates of infusion into the foetal amniotic sac induced delivery with a shorter latency. Maternal infusion of 1.24 mg cortisol/h produced parturition providing it was continued for at least 24 h. If after this period the infusion was terminated, delivery still occurred after a further 48 h (i.e. 72 h in all). Progesterone administration to the mother blocked the effect of cortisol administered by the maternal or fetal route.