Newborn male and female rats aged between 24 and 48 hours were pinealectomized (Px) or sham-operated (Sh). Animals were sacrificed at 45 and 60 days of age, at the trough and peak of the adrenocortical diurnal cycle. Blood was collected for corticosterone determinations and the adrenal glands weighed. Adrenal gland weight was greater at 45 days (p less than 0.01) in Px rats than in Sh rats. This difference in adrenal gland weight disappeared in the group sacrificed 60 days post-operatively. On the other hand, the diurnal plasma rhythm of corticosterone remained intact in male and female rats with or without pineal at 45 and 60 days. Furthermore, the trough and peak plasma corticosterone levels were not different in Px 45 and 60 days post-operatively. These data do not support the assumption of the important role of the pineal in the newborn rat for the maintenance of a normal circadian rhythm of the pituitary-adrenal system.