In neonatal rat gonadotropes, melatonin inhibits gonadotropin-releasing-hormone (GnRH)-stimulated increase in intracellular Ca<sup>2+</sup> concentration ([Ca<sup>2+</sup>]<sub>i</sub>); in cells transfected with the Mel1a melatonin receptor, however, melatonin has been shown to potentiate agonist-stimulated [Ca<sup>2+</sup>]<sub>i</sub> increase. To elucidate this discrepancy, we investigated the effects of melatonin in neonatal gonadotropes over a wide range of melatonin concentrations. Nystatin perforated patch recording of Ca<sup>2+</sup>-dependent potassium currents was used to monitor GnRH-induced [Ca<sup>2+</sup>]<sub>i</sub> changes. In 32% of cells, increasing melatonin concentrations in the range of 1 p M to 100 n M prolonged the latency of, and inhibited GnRH (10 n M)-stimulated [Ca<sup>2+</sup>]<sub>i</sub> increases in a concentration-dependent manner. In the remaining 68% of cells, the Ca<sup>2+</sup> increase elicited by exposure to 10 n M GnRH was also inhibited by picomolar concentrations of melatonin, but at nanomolar concentrations the inhibitory effect disappeared and melatonin was only able to prolong the latency of the response. This dual effect of melatonin however was not observed in cells stimulated with lower (2 n M) GnRH concentrations; in that case, melatonin was inhibitory at all concentrations tested with an IC<sub>50</sub> of about 30 p M. In contrast, application of nanomolar concentrations of melatonin resulted in potentiation of the GnRH-induced Ca<sup>2+</sup> increase in a small population of gonadotropes which did not respond by inhibition or prolonged latency. These results indicate that in neonatal gonadotropes, melatonin has both inhibitory and potentiating effects on GnRH-stimulated [Ca<sup>2+</sup>]<sub>i</sub> increases. Ranges of concentrations needed to produce either effect suggest that two distinct G proteins may be involved, as already observed in transfected cells.