Expression of mRNAs coding for the ACTH secretagogues corticotropin- releasing factor (CRF) and arginine vasopressin (AVP) was examined in the hypothalamic paraventricular nucleus (PVN) of rats bearing hippocampal lesions. Either total hippocampectomy (HPX) or extirpation of the dorsal hippocampus (DHPX) precipitated a 4-fold increase in CRF mRNA expression relative to sham-operated controls (SHAM), as determined by semiquantitative in situ hybridization histochemistry. AVP mRNA was localized to individual parvocellular neurons of the medial parvocellular division of the PVN in only the HPX and DHPX groups, consistent with enhanced production of AVP message in this neuronal population subsequent to hippocampal damage. HPX did not affect AVP mRNA content in magnocellular divisions of PVN. Plasma beta- endorphin levels were significantly elevated in the HPX and DHPX groups relative to SHAM animals, indicating a chronic increase in release of proopiomelanocortin peptides from the anterior pituitary gland in response to hippocampal lesion. Circulating corticosterone levels were elevated in HPX rats as well. To control for effects of lesion size and location, additional animals received large ablations of cerebral cortex or cerebellum. In neither case was CRF or AVP mRNA significantly altered in the PVN. The results suggest that the hippocampus exercises a tonic inhibitory role on ACTH secretagogue production in neuroendocrine neurons promoting ACTH release.