The effect of corticotropin-releasing factor (CRF) on the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator, the central neuronal system governing pulsatile pituitary luteinizing hormone (LH) secretion, was studied electrophysiologically in 6 ovariectomized rhesus monkeys bearing bilateral arrays of recording elecrodes implanted in the mediobasal hypothalamus. ‘Volleys’ of increased multiunit activity (MUA) were recorded for 6–10 h in animals placed in primate chairs. The circulating concentrations of LH and cortisol were determined by radioimmunoassay in blood samples taken every 10 min for 3–4 h prior to the administration of CRF (200 µg, i.v.) and for 3–6 h thereafter. CRF resulted in a significant decrease in the frequency of pulse generator activity in 4 of 6 animals, a significant decrease in the duration of MUA volleys and a rise in circulating cortisol levels in all 6 monkeys. Treatment with metyrapone (30 mg/kg, i.m.), an inhibitor of adrenal steroidogenesis that prevented the CRF-induced rise in serum cortisol levels, did not reverse the inhibitory effects of CRF on the frequency or duration of MUA volleys. The opiate antagonist naloxone (0.8 mg/kg, i.v., 10 min prior to CRF followed by 0.8 mg/kg/h infusion) blocked the effects of CRF on MUA volley frequency in 2 of 3 animals, but failed to block the effect of CRF on MUA volley duration, suggesting that endogenous opioids may mediate the action of CRF on pulse generator frequency but not o