The quality of metaphase II oocytes deteriorates rapidly following ovulation as the result of an aging process associated with impaired fertilizing potential, disrupted developmental competence, and increased likelihood of embryonic resorption. Because oxidative stress accelerates the onset of apoptosis in oocytes and influences their capacity for fertilization, this study aimed to characterize the significance of such stress in the postovulatory aging of mouse oocytes in vitro. We investigated the ability of the potent antioxidant melatonin to arrest the aging process when used to supplement oocyte culture medium. This study demonstrated that oxidative stress may occur in oocytes after as little as 8 h in culture and coincides with the appearance of early apoptotic markers such as phosphatidylserine externalization, followed 16 h later by caspase activation (P < 0.05) and morphological evidence of oocyte senescence. Importantly, supplementation of oocyte culture medium with 1 mM melatonin was able to significantly relieve the time-dependent appearance of oxidative stress in oocytes (P < 0.05) and, as a result, significantly delay the onset of apoptosis (P < 0.05). Furthermore, melatonin supplementation extended the optimal window for fertilization of oocytes aged for 8 and 16 h in vitro (P < 0.05) and significantly improved the quality of the resulting embryos (P < 0.01). We conclude that melatonin may be a useful tool in a clinical setting to prevent the time-dependent deterioration of oocyte quality following prolonged culture in vitro.