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Background & Aims: Change in visual experience impairs circadian rhythms. In this study, The effects of visual deprivation during critical period of brain development and melatonin intake on synaptic plasticity of hippocampal CA1 neurons were evaluated. Methods: This experimental study was done on male rats kept in standard 12 hour light/dark condition (Light Reared-LR) or in complete darkness (Dark Reared-DR). Each group, was divided into 3 sub groups of 2, 4 and 6 week old rats (n=16). Excitatory post synaptic field potentials were recorded from dendrite of CA1 area neurons. Then, half of animals in each group received melatonin via ICV and field potentials recording was repeated. Finally, using tetanic stimulation, Long-term potentiation (LTP) was induced. Results: The amplitude of basic responses of the LR animals decreased with age incaease (P<0.01). Visual deprivation and also melatonin increased the amplitude of basic responses of the DR group (P<0.01), and inhibited LTP induction in CA1 circuits. In comparison to the LR group, melatonin injection had less destructive effect on the LTP induction in the DR group (P<0.001). Conclusion: Visual deprivation during critical period of brain development increases basic responses of rat’s CA1 neurons and inhibits LTP induction in these neurons, time dependently. Also, the visual deprivation opposes the destructive effects of melatonin on LTP induction in CA1 neurons in a time-dependent manner.