Previous results indicate that GH-releasing factor (GRF) induces a dose-related stimulation of somatostatin (SRIF) release from median eminence fragments incubated in vitro. In the present investigation we examined whether this action was mediated by other neurotransmitters or neuromodulators. Studies using receptor blockers for dopamine (pimozide), alpha-adrenergic receptors (phentolamine), and muscarinic cholinergic receptors (atropine) revealed that these receptor blockers, at a dose of 10(-6) M, which was capable of blocking the response to the relevant transmitter in previous studies, had no effect on basal release of SRIF in the static incubation system and failed to modify the response to GRF (10(-10) M). On the other hand, the opiate receptor blocker naloxone at a dose of 10(-6) M, although failing to alter basal release, completely blocked the response to 10(-10) M GRF. To determine the opioid peptide involved in mediating the SRIF release induced by GRF, highly specific antibodies directed against beta-endorphin were added to the in vitro incubation system. These antibodies significantly depressed basal release SRIF and completely blocked the response to 10(-10) M GRF. Incubations in the presence of normal rabbit serum or highly specific antiserum directed against alpha MSH had no effect on either basal release of SRIF or that induced by GRF. These results suggest that in this incubation system there is a beta-endorphin tone which is partially responsible for the basal release of SRIF and that the stimulation of SRIF release induced by GRF is mediated via beta-endorphin terminals, which presumably synapse on the terminals of the somatostatinergic neurons in the median eminence fragment.