Pulsatile release of luteinizing hormone (LH) is governed by pulsatile release of LH-releasing hormone (LHRH), and each LHRH pulse is produced by synchronized firing of LHRH neurons. However, the mechanism of this synchronized firing is unknown. An increase (volley) in the multiunit activity (MUA), at 20- to 30-min intervals, accompanied by an LH pulse, is found in the median eminence (ME) of ovariectomized rats, suggesting that the MUA reflects the activity of LHRH neurons directly or indirectly. On the hypothesis that LHRH released from LHRH neurons locally in the hypothalamus was involved in the mechanism responsible for the synchronized firings of LHRH neurons, we examined the effect of exogenously administered LHRH on the MUA showing volleys in association with pulsatile LH release in ovariectomized rats. Intravenous injection of LHRH (50 micrograms/kg) evoked a premature volley (average 5.0 min after the injection). Microinjection of LHRH (1 ng) into the ME also evoked a premature volley (average 4.6 min after the injection), accompanied by an LH pulse. Microinjection of LHRH (1 ng) into the medial preoptic area did not cause these effects. The results suggest that LHRH released from LHRH neurons locally in the hypothalamus somehow activates its own and neighboring LHRH neurons, causing synchronized firings of LHRH neurons, and that this mechanism exists at least in and around the ME.