In order to investigate the physiological role of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased GABA_C receptor activity, as demonstrated by the ability of the GABA_C receptors antagonists picrotoxin and TPMPA to abolish the effects of melatonin. In contrast, neither the GABA_A receptor antagonist bicuculline nor the GABA_B receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the three known melatonin receptors, MT1 (Mel_1a), MT2 (Mel_1b) and Mel_1c. Since the effect of melatonin on tectal calcium increases was antagonized by 4-P-PDOT, a MT2 selective antagonist, we performed western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting via MT2 receptors, modulates GABA_C receptor activity in the optic tectum and that this effect is influenced by the light/dark cycle.