AbstractMice given a single large dose of corticosterone at 2 days of age, and tested when adult, have shown lasting alterations in operant behavior, associated with reductions in cerebral weight and DNA content. When mice were given corticosterone at 22 days, no changes appeared in cerebral weight, DNA, or in operant behavior, but an apparent decrease did occur in open‐field activity. The results suggest that the effects of corticosterone treatment at 2 days on the developing brain are mediated by one or more types of sensitivity to the steroid that have been largely lost by 22 day