Melatonin may modulate retinal information processing through activating specific receptors coupled to GTP-binding protein.In the present work we studied effects of melatonin on ON type bipolar cells (BCs).Intracellular recordings showed that melatonin depolarized ON type BCs driven by rod signal (Rod-ON-BCs) and significantly suppressed their light responses,which was reversed by the MT2 receptor specific antagonist 4-phenyl-2-propionamido-tetralin (4-P-PDOT).In contrast,melatonin was without effect on ON type BCs driven by cone signal (Cone-ON-BCs).Consistent with the intracellular recordings,melatonin suppressed the scotopic electroretinographic (ERG) b wave,but not the photopic b wave,by activating the MT2 receptor.Melatonin did not change the scotopic ERG PⅢ component,suggesting a direct action of melatonin on the Rod-ON-BCs.