The mechanism(s) by which melatonin (MEL) can regulate gonadotropin secretion remains unresolved. Accordingly, we used acute in vitro incubations of male rat hypothalamic tissues to investigate effects of MEL on hypothalamic gonadotropin-releasing hormone (GnRH) secretion. At 10:00 h (3.5 after lights on), addition of 1 nM MEL to the medium inhibited (p = 0.028) GnRH release from the median eminence (ME) by 24%, 100 nM MEL had no significant effect on GnRH release, and 10 microM MEL tended to inhibit (p = 0.056, 21%) GnRH release. At 15:00 h, none of these MEL dosages significantly altered GnRH release from the ME. When the effect of a lower range of MEL dosages was evaluated, 1 nM MEL again inhibited (24%, p = 0.032) GnRH release from the ME at 10:00 h, and a linear dose-response relationship between initially increasing dosages (0, 0.01, 0.1, 1 nM) of MEL treatment and decreasing GnRH release was evident (r = 0.88), although a further 10 fold increase in MEL dosage (10 nM) resulted in a loss of suppression by MEL. Treatment with these dosages of MEL did not significantly alter ME GnRH release at 15:00 h. In contrast to the results with the ME alone, treatment with 0.1 microM MEL increased (p = 0.018) GnRH release from the arcuate-median eminence region (ARC-ME) by 60.2% at 10:00 h, whereas treatment with either 0.001 or 10 microM MEL did not significantly affect GnRH release. Treatment with these dosages of MEL did not significantly alter ARC-ME GnRH release at 15:00 h.(ABSTRACT TRUNCATED AT 250 WORDS)