Plasma cortisol levels are raised in the cord blood of newborn human infants (Migeon, Prystowsky, Grumbach & Byron, 1956). In man these high levels have been attributed to placental permeability to the high maternal levels of cortisol which occur during parturition. In the pregnant sheep the placenta does not appear to be permeable to cortisol in the latter stages of gestation since variations in maternal cortisol levels are not reflected in changes in the level of cortisol in the foetal blood (Bassett & Thorburn, 1969). Few observations have been made in this species of any changes in placental permeability to cortisol that may occur during parturition. Studies with radioactive cortisol suggest that there is little, if any, increase in the passage of cortisol across the placenta in either direction during parturition (Beitins, Kowarski, Shermeta, De Lemos & Migeon, 1970; Comline, Nathanielsz, Paisey & Silver, 1970). The hypothalamo—adenohypophysial—adrenocortical system is active