Serum and adrenal corticosterone was measured by competitive protein-binding radioassay in rats subjected to saline injection, ACTH administration or ether fumes. Groups of rats were tested at 5, 7, 9, 11, 13, 15, 20 and 25 days of age and measurement of hormone level was made either before treatment, to obtain basal values, or 15 min after treatment. Furthermore, the time-course of corticosterone release after ether was determined in 9- and 15-day-old rats. Neonatal rats responded to ether exposure, ACTH administration or saline injection with a significant rise in serum and adrenal corticosterone concentration above basal levels as early as 5 days of age. By 9 days of age, response to stress was qualitatively the same as that of the 25-day-old rat. The time course of adrenal responsiveness to ether stress was similar in 9- and 15-day-old rats, both age groups showing significant increases in hormone concentration by 15 min. These results contradict the concept of the ‘stress non-responsive period’ which was promoted by previous studies based on the nuorometric analysis of corticoster