We have investigated the effect of melatonin administration on the cytometric and endocrine functions of the ovary during aging. Young cyclic (3 months old), middle-aged pre-acyclic (13 months old), and old acyclic (22 months old) female Wistar rats were used for two months, in both control and melatonin-treated groups. Cell cycle by flow cytometry: the percentage of ovarian cells in the G0-G1 phase was the highest in both control and melatonin-treated rats. However, melatonin treatment significantly reduced (P< 0.05) the percentage of cells in the G0-G1 phase compared to control rats. This reduction of cells in the G0-G1 phase is derived to the S phase in cyclic and acyclic rats. The possibility that a tumoral process leads to a proliferative effect observed in young and acyclic melatonin-treated rats was ruled out because no significant differences were found for p53 and Ki67 expression levels between control and melatonin groups. Density of oocytes: the oocyte number per ovary unit volume was not affected by melatonin treatment in the three age ranges studied. Melatonin treatment in middle-aged (pre-acyclic) rats resulted in significantly higher (P< 0.05) ovarian volume; higher oocyte volume, without significant differences, and oocytes in circular form were significantly (P< 0.05) higher than in control rats. Melatonin treatment during pre-acyclic age range could resynchronize the estrous cycle periodicity. Melatonin treatment was able to maintain the same levels of estradiol in the pre-acyclic age groups studied as those observed in the young cyclic rats. The present results indicate that melatonin administration to middle-aged female rats produces beneficial effects that extend the reproductive function of the ovary.