Parturition in sheep is initiated by a sharp rise in the rate of secretion of cortisol by the fetal adrenal. Increased secretion is due partly to enhanced responsiveness to corticotropin (ACTH) and partly to increased fetal concentrations of corticotropin. Cortisol acts on placental enzymes active in the biosynthesis of oestrogens from progesterone. Thus placental secretion of oestrogen increases and that of progesterone decreases. This change in the ratio of oestrogen: progesterone, particularly the rise in oestrogen, stimulates release of prostaglandin F2alpha (PGF2alpha) from the maternal placenta and to a lesser extent from the myometrium. PGF2alpha enhances the myometrial response to oxytocin and, after a latent period, stimulates contractions. The onset of parturition is normally associated with softening of the cervix, the mechanism of which is uncertain. Uterine contractions in the presence of a distensible cervix lead to parturition.