There is considerable evidence that CRF-containing neurons integrate the endocrine, autonomic, immune, and behavioral responses to stress. In this study we examined long term effects of early stress on developing hypothalamic and extrahypothalamic CRF neural systems in the rat brain and subsequent responses to stress in the adult. Specifically, we sought to determine whether adult male rats previously isolated for 6 h daily during postnatal days 2-20 react in a biochemically distinct manner to a mild foot shock stress compared to controls. Four treatment groups were examined: nondeprived (NDEP)/no shock, NDEP/shock, deprived (DEP)/no shock, and DEP/shock. Compared to the NDEP group, DEP rats exhibited an increase in both basal and stress-induced ACTH concentrations. Moreover, DEP rats exhibited a 125% increase in immunoreactive CRF concentrations in the median eminence and a reduction in the density of CRF receptor binding in the anterior pituitary compared to those in all NDEP rats. Alterations in extrahypothalamic CRF systems were also apparent in DEP vs. NDEP animals, with an observed 59% increase in the number of CRF receptor-binding sites in the raphe nucleus and an 86% increase in immunoreactive CRF concentrations in the parabrachial nucleus. These results indicate that maternal deprivation before weaning in male rats produces effects on CRF neural systems in both the central nervous system and pituitary that are apparent several months later and are probably associated with persistent alterations in behavioral response in adult rats.