Adrenocorticotropic hormone (ACTH) has been frequently used as an anticonvulsant drug in some childhood seizure disorders. Despite its widespread use, few studies have evaluated the effects of ACTH on seizure susceptibility in the developing animal or the long-term consequences of ACTH treatment on brain development. In this study, ACTH was given either for 2 days or 14 days prior to kindling in 15-, 22-, and 30-day-old rats. Morphological changes in the brain were studied using routine light microscopy and dendrite branch counting following Golgi staining. Both acute and chronic ACTH treatment inhibited the rate of kindling in all three age groups. There were no differences in brain morphology between the controls and the ACTH-treated rats killed shortly after kindling. Rats treated with ACTH and killed as adults, however, had significantly more dendrite branches than did controls. In the immature brain, ACTH treatment reduces seizure susceptibility and has no long-term deleterious effects on neuronal growth.