[Postnatal development of the adrenal gland in the rat (author's transl)]

Postnatal development of adrenocortical function of rats was studied. During the first week after birth the increase in adrenal weight was very slight. Consequently, the relative weight of the gland per body weight decreased progressively during the postnatal 14 days. However, adrenal corticosterone concentration showed a gradual elevation over this period of postnatal days. Adrenal corticosterone response to ether stress did not appear in male rats at 12 days of age, but a distinct response was observed in female ones at the same age. A sex difference in the duration of the stress-nonresponsive period is likely to exist in newborn rats. In accord with the stress-nonresponsive period, the thymus-lymphatic system showed a marked development. The pattern of circadian periodicity of plasma corticosterone levels in 21-day-old rats was somewhat different from that in adult rats. Peak value of the corticosterone rhythm was attained at 0300 in the infant rats. A sex difference was again noticed in the evening rise of plasma corticosterone level in these infant rats: the rise was steeper in females than in males. A significant circadian periodicity was observed in female rats at 14 days of age, but the periodicity was equivocal in male ones at the same age.