Corticotropin-releasing factor initially stimulates the release of beta-endorphin and dynorphin from rat hypothalamic slices in vitro; with time, in the continued presence of corticotropin-releasing factor, the release of both these peptides declines. The studies described here were undertaken to test whether this decline could be due to the operation of inhibitory feedback mechanisms associated with the function of the opioidergic neurons. When the opioid receptor antagonist naloxone was added to the superfusion medium in the presence of corticotropin-releasing factor, the time-related decrease in opioid release was not observed. Potassium ions also caused an increase, followed by a decrease, in opioid peptide release, and naloxone also prevented the latter from occurring. In addition, naloxone on its own, produced a Ca2+-dependent increase in the non-stimulated release of beta-endorphin and dynorphin, and this action was resistant to tetrodotoxin. These findings suggest that opioid receptors mediate inhibitory feedback effects upon the secretory activity of beta-endorphin and dynorphin neurons in the hypothalamus.