Abstract The development of daily changes in serum corticosterone (B) values was followed in maturing Sprague-Dawley derived rats raised from birth on a 14 hour light:10 hour dark cycle or received from the breeder on the day of weaning, and placed in a 14:10 cycle, In shipped rats., initial values of B were high at 8 AM on day 22 but had dropped by day 26. Evening values of B(8 PM) were similar in 22 and 26 day old rats. A significant difference between the noon and 8 pm values of B was obtained at 18 days of age in females and 19 days of age in males. Samples taken every 4 hours on days 19 and 20, before weaning, revealed a daily pattern similar to that of adults except that high values were obtained at 8 AM on day 19 and on day 20 at the time of a trough period in older rats. The evening peak was also longer in duration, failing to fall at midnight as in 26 day old rats. It is concluded that the pattern of the adrenal rhythm matures over a three or four day period in the preweanling rat.