The objective of the present study was to examine the effects of melatonin on transcripts of isoforms of calcium/calmodulin-dependent protein kinases in rat insulinoma beta-cells INS-1. Investigations show that calcium/calmodulin-dependent kinase IV and calcium/calmodulin-dependent kinase 2d are expressed in human and rat pancreatic islets and INS-1 cells. By application of either forskolin or 3-isobutyl-1-methylxanthine for 6 hours, calcium spiking was evoked and the release of insulin was increased. The expression of the calcium/calmodulin-dependent kinase IV and calcium/calmodulin-dependent kinase 2d transcripts was significantly increased due to forskolin or 3-isobutyl-1-methylxanthine. Acute melatonin treatment (6 h) in the presence of either forskolin or 3-isobutyl-1-methylxanthine caused a significant decrease in insulin release and induced significant downregulation of calcium/calmodulin-dependent kinase IV and calcium/calmodulin-dependent kinase 2d transcripts in INS-1 batch cultures. The attenuating effect of melatonin on transcripts could be almost completely reversed by preincubation with the melatonin receptor antagonist luzindole. Thus, the insulin-inhibiting effect of melatonin in INS-1 cells is associated with significant changes in transcripts of calcium-signaling components suggesting that melatonin influences gene expression of components, which are known to be involved in insulin secretion or insulin gene expression.