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John Radcliffe Hospital and Nuffield Institute for Medical Research, Headley Way, Headington, Oxford, 0X3 9DU

(Received 1 September 1976)

Liggins, Fairclough, Grieves, Kendall & Knox (1973) drew attention to the role of the foetal pituitary-adrenal system in the initiation of parturition in sheep by demonstrating prolonged pregnancy after foetal hypophysectomy and premature delivery after intra-foetal infusion of glucocorticoid or synthetic corticotrophin (ACTH). Bassett & Thorburn (1969) observed a rise of foetal cortisol before spontaneous parturition in this species. The increasing foetal cortisol induces the decline in maternal plasma progesterone observed in the last 10-15 days of pregnancy and the oestradiol increase in the last 24 h before delivery (see Thorburn, Challis & Robinson, 1976). In naturally occurring prolonged pregnancy in cows (Holm & Short, 1962) and sheep (Basson, Morgenthal, Bilbrough, Marais, Kruger & van der Merwe, 1969) the concentration of progesterone in maternal plasma remains raised after normal term. Similarly,