Abstract: Several studies have revealed that melatonin affects the insulin secretion via MT1 and MT2 receptor isoforms. Owing to the lack of selective MT1 receptor antagonists, we used RNA interference technology to generate an MT1 knockdown in a clonal β-cell line to evaluate whether melatonin modulates insulin secretion specifically via the MT1 receptor. Incubation experiments were carried out, and the insulin concentration in supernatants was measured using a radioimmunoassay. Furthermore, the intracellular cAMP was determined using an enzyme-linked immunosorbent assay. Real-time RT-PCR indicated that MT1 knockdown resulted in a significant increase in the rIns1 mRNA and a significantly elevated basal insulin secretion of INS-1 cells. Incubation with melatonin decreased the amount of glucagon-like peptide 1 or inhibited the glucagon-stimulated insulin release of INS-1 cells, while, in MT1-knockdown cells, no melatonin-induced reduction in insulin secretion could be found. No decrease in 3-isobutyl-1-methylxanthine-stimulated intracellular cAMP in rMT1-knockdown cells was detectable after treatment with melatonin either, and immunocytochemistry proved that MT1 knockdown abolished phosphorylation of cAMP-response-element-binding protein. In contrast to the INS-1 cells, preincubation with melatonin did not sensitize the insulin secretion of rMT1-knockdown cells. We also monitored insulin secretion from isolated islets of wild-type and melatonin-receptor knockout mice ex vivo. In islets of wild-type mice, melatonin treatment resulted in a decrease in insulin release, whereas melatonin treatment of islets from MT1 knockout and MT1/2 double-knockout mice did not show a significant effect. The data indicate that melatonin inhibits insulin secretion, primarily via the MT1 receptor in rat INS-1 cells and isolated mouse islets.