Prolonged pregnancy following foetal hypophysectomy or adrenalectomy (Liggins, Kennedy & Holm, 1967; Drost & Holm, 1968) and premature parturition following injection of corticotrophin or corticosteroids into the ovine foetus (Van Rensburg, 1967; Halliday & Buttle, 1968; Liggins, 1968) implicate the foetal adrenal cortex in the initiation of parturition in sheep. Consequently, foetal plasma corticosteroid concentrations and corticosteroid secretion by the foetal adrenal should be greatly increased towards term. Observations during acute studies on anaesthetized sheep suggest this (Alexander, Britton, James, Nixon, Parker, Wintour & Wright, 1968), but there is no information about blood concentrations or secretion rates of corticosteroids in normal undisturbed sheep foetuses in utero. We cannulated the carotid artery and the facial branch of the jugular vein of four single Merino foetuses with polyvinyl chloride tubing (o.d. 1·27 mm., i.d. 0·86 mm.) with the ewes under general anaesthesia (pentobarbitone sodium (20 mg./kg., i.v.) followed by halothane and oxygen).