Abstract: In this study, we examined whether melatonin treatment would increase new cell formation in the hippocampus in ovariectomized (OVX) mice. Chronic exogenous melatonin administration increased bromodeoxyuridine (BrdU) (OVX-sham 72 ± 3.2 versus OVX-mel 122 ± 12.0; P < 0.05) and doublecortin (DCX) (OVX-sham 88 ± 3.1 versus OVX-mel 176 ± 9.9; P < 0.05) immunoreactive cells in the hippocampus of ovariectomized mice. This neuronal development was correlated with synaptic plasticity, identified using the Golgi impregnation method to quantify dendritic spines in mouse dentate gyrus (DG). Finally, the antidepressant-like state of the animals was evaluated by the tail suspension test. The results indicate that melatonin acts on birth, survival, and differentiation of new neurons in the hippocampus, stimulates maturation of spines, and exerts an antidepressant-like action under estrogen-deprived conditions, in both a strain- and gender-independent manner, suggesting that this indoleamine may be useful in improving brain functions.