The purpose of the present investigation was to determine the effects of naloxone on LHRH and catecholamine release from preoptic area-medial basal hypothalamic (POA-MBH) fragments in vitro. Ovariectomized rats were treated with estradiol benzoate, followed 2 days later by progesterone. The rats were killed 2 after progesterone administration and the POA-MBH dissected out and incubated in a perifusion system. After preincubation, medium with or without naloxone (1 mg/ml) was infused into the perifusion chambers and LHRH, norepinephrine, epinephrine and dopamine release were monitored. Naloxone concurrently released LHRH and the three catecholamines during the entire perifusion period. LHRH and catecholamine output returned to the control range within 30–45 min after cessation of naloxone infusion. These data show that naloxone can promptly stimulate catecholamine release and concur with the view that LH release evoked by naloxone in vivo may be due to hypersecretion of LHRH and they further raise the possibility that norepinephrine and epinephrine released in the vicinity of peptidergic neurons in the POA-MBH may participate in LHRH hypersecretio