OBJECTIVE We evaluated the effect of melatonin, as a potent antioxidant agent, on glutathione (GSH) and reactive oxygen species (ROS) levels, as well as histone H3 lysine 9 (H3K9), and H4 lysine 12 (H4K12) acetylation when added to oocytes culture medium. METHODS In this experimental study, two in vitro and in vivo groups were used. In the in vitro group, cumulus oocyte complexes (COCs) from the ovaries of B6D2F1 mice were cultured in maturation medium containing two doses of melatonin (10-9 and 10-6 M) and without melatonin [control group treated with dimethyl sulfoxide (DMSO)] for 22-24 hour. The cumulus expansion and nuclear status were monitored by an inverted microscope. Next, COCs were isolated from the oviducts of superovulated mice and studied as the in vivo group. In in vitro and in vivo matured oocytes, GSH and ROS levels were assessed by monochlorobimane (MCB) and 2-7-dichlorodihydrofluorescein diacetate (H2DCFDA) staining, respectively. Changes in histone acetylation were examined by immunofluorescent staining with specific antibodies against acetylated H3K9 and H4K12. RESULTS The H4K12 acetylation and ROS levels were significantly higher in the oocytes matured in the in vitro group compared to the in vivo group (P<0.05). Furthermore, glutathione levels in the in vitro group were considerably lower than that of the in vivo group (P<0.05). Melatonin at the concentration of 10-6 M had the most substantial effect on nuclear maturation and histone acetylation as well as glutathione and ROS levels in the in vitro group (P<0.05). CONCLUSIONS Exogenous melatonin improves the competence of mouse oocytes during in vitro maturation (IVM).