The expanding perinatal use of glucocorticoids entails potentially hazardous effects of these hormones on nervous system development. Neonatal animal experimentation with pharmacological doses of glucocorticoids has revealed immediate effects on brain cell division, differentiation, myelination, and electrophysiological reactions. In addition, delayed (latent) effects include changes in control of circadian periodicity, altered biogenic amine levels, altered response to stress, and changes in ultimate behavior. Thus perinatal hormone therapy during critical periods of brain development is capable of exerting irreversible immediate effects on brain cell division and differentiation, resulting in latent or long-term physiological and behavioral effects.