24 and 28 day old rats, raised in a 14 h light: 10 h dark (14:10) cycle, showed a clear serum corticosterone (B) peak at the beginning of the dark period (18.00 h) with an amplitude of 25–30 µg%. Other B values during the 24 h period were lower (9–12 µg%). Rats raised in a 8.5:15.5 cycle had a B peak (after onset of the dark period, at 22.00 h). Rats raised in constant light (LL) showed no variation in B throughout the 24 h. Weanling rats raised in a 8.5 :15.5 cycle appeared to synchronize more rapidly to a 14:10 cycle than did rats raised in LL. When placed in LL at weaning, rats raised either in 14:10 or 8.5:15.5 had lost their periodicity by day 24. When rats raised in 8.5:15.5 were shifted to LL or 14:10 after weaning age (day 24), the pattern of response was different. One day after the shift, rats placed in a 14:10 cycle displayed a pattern of B synchronized to the normal 14:10 pattern for rats born and raised in that light cycle. This synchrony persisted for at least 3 days. Rats placed in 14:10 at 21 days of age did not show a fully synchronized pattern until day 28. It can be concluded that weanling rats exposed to a light-dark cycle adapt quickly to a new light cycle and that older prepubertal rats shift even more quickly.