Corticotropin-releasing factor (CRF) has been shown to release endogenous opioid peptides from several rat brain regions. Since we have demonstrated previously that the actions produced by intrathecally administered CRF in mice involve spinal kappa opioid receptors, experiments were conducted in this study to test the possibility that CRF may release dynorphin A, a putative endogenous kappa opioid agonist, from the mouse spinal cord. Using a superfusion system in vitro, mouse spinal cords were superfused with aerated (95% O2, 5% CO2) Krebs-Ringer buffer. Fractions of superfusion were collected and dynorphin A levels in each fraction were monitored by radioimmunoassay. The presence of CRF in the perfusion buffer stimulated significantly the release of immunoreactive dynorphin A. The releasing rate of immunoreactive dynorphin A returned to the basal level after withdrawing CRF from the superfusion buffer. The stimulatory effect of CRF on the release of immunoreactive dynorphin A was abolished by alpha-helical CRF-(9-41), a CRF receptor antagonist, indicating that the dynorphin-releasing effect of CRF was mediated by CRF receptors in the spinal cord. Also the dynorphin-releasing effect of CRF was a concentration-related phenomenon, with an estimated EC50 value of 5.3 nM. The results from this study support the hypothesis that intrathecally administered CRF may produce its effects by releasing endogenous dynorphin from the terminals of dynorphin-containing neurons in the spinal cord. This study also provides evidence to support the notion that there is a close communication between CRF- and opioid peptide-containing neuronal pathways in the central nervous system.