The evolution of adrenal weight, of the level of ascorbic acid and the levels of adrenal and plasma corticosterone were measured in the newborn rat at the moment of birth and during the hours that followed. Between 0 and 4 h, the absolute and relative weights of the adrenals decreased by 14 and 10%, respectively, and were even lower at 24 h. The amount of ascorbic acid in the adrenals decreases by 17% between 0 and 4 hours. Adrenal corticosterone levels dropped abruptly during the first hour of life, accompanied by an increase in the plasma levels of this hormone. These results suggest that during the initial hours of life, there is a significant stimulation of the adrenal cortex, probably connected to birth-related stress phenomena. An encephalectomy, leaving the pituitary in situ, or decapitation of the rat fetus during the last days of gestation impair the growth and endocrine activity of the adrenal glands. Normal adrenocortical function is restored by injecting hypothalamic extracts or ACTH at the time of surgery (9, 16). At the end of the gestation period, rat fetuses respond to such stimuli as epinephrine or formalin injection by depleting adrenal ascorbic acid levels 1 h after the stress (5, 24). The administration of ether, or laparotomy plus ether, on day 20 of gestation induce an immediate rise of fetal hypothalamic CRF and adrenal corticosterone (12, 27). All these results afford good evidence for an autonomous hypothalamic-hypophyseal-adrenocortical axis during the last days of gestation of the rat fetus. The purpose of the present report was to discover if the adrenal cortex was activated by the stress of labor and birth, as well as by adaptation of the newborn animal to new environmental conditions. The parameters examined were adrenal weight, the levels of adrenal ascorbic acid and corticosterone and the circulating corticosterone levels during the hours following birth.