The regulation of adrenal activity during the perinatal period raises different experimental problems; studies are often limited and their conclusions vary with the species studied. During the perinatal period, the profile of the ratio of adrenal weight to body weight reaches a minimum earlier the more mature is the species; this minimum occurs before birth in sheep, at birth in guinea-pigs and 10 days after birth in mice. In mature species, fetal plasma cortisol is higher than that of the mother; it rises sharply near term. In other species, maternal plasma cortisol remains higher than that of the fetus which also rises during the days before birth. In all species, the fetal adrenal is activated concomitantly with a sharp increase of the action of corticosteroid binding globulin on fetal plasma. The origins of this protein in the fetal plasma and the regulation of its concentration are topics of actual research. Before birth, placental transfer of cortisol increases regularly and slowly. During the perinatal period it is demonstrated that fetal catabolism of cortisol augments up to the adult level independently of changes in hormone secretion around birth.