The effects of calcium depletion and of three calcium channel drugs on melatonin synthesis in pineal organs of rainbow trout (Oncorhyncus mykiss) were examined. Dark-induced melatonin synthesis was inhibited by calcium depletion, by treatment with nitrendipine (NTR), an antagonist of the L-type voltage-sensitive calcium channel, and by treatment with omega-Conotoxin GVIA, an antagonist of the N-type voltage-sensitive calcium channel. Bay K 8644, an agonist of the L-type channel, had no significant effect on pineal melatonin synthesis. These data represent evidence that calcium entry into trout pineal photoreceptor cells through voltage-sensitive calcium channels is important in the maintenance of dark-induced melatonin synthesis and that light's inhibitory effect on melatonin synthesis may be mediated by closure of these channels. NTR-induced inhibition of dark-induced melatonin synthesis was abolished when dibutyryl cyclic AMP (db-cAMP) was administered to NTR-treated pineal organs, suggesting that calcium acts upstream of cAMP in regulating melatonin synthesis.