OBJECTIVE In this study, we investigated the effect of melatonin on fracture healing in the rat tibia model by using biochemical and histopathologic methods. METHODS In this study 80 male Sprague-Dawley rats were randomized into two groups, a control group (Group 1) and melatonin group (Group 2) with eight rats per group according to the day of sacrifice (Days 1, 3, 7, 14 and 28). The fractures were produced by the manual breakage using plate-bending devices, placed at the distal 3(rd) of the right tibia. Group 2 received 30 mg/kg/day melatonin and group 1 1% alcohol in saline 5 ml/kg/day intraperitoneally during the experiment. Plasma Malondialdehyde (MDA) levels, activity of superoxide dismutase (SOD) and myeloperoxidase (MPO) were measured biochemically. The fracture healing was evaluated using a five-point scale defined by Allen et al. RESULTS Malondialdehyde levels in group 2 decreased at days 3, 7, 14, and 28 compared to control values (p<0.05). Superoxide dismutase activity in group 2 decreased at days 3, 7 and 14, and returned to the 1(st) day value after 28 days. Myeloperoxidase values in group 2 decreased at days 1, 3, and 7 (p<0.001). Histopathological specimens of healed tibias showed two animals with complete cartilaginous union, five with incomplete bony union and one with complete bony union in the group 2. In contrast, in the group 1, six rats showed complete cartilaginous union and two showed incomplete cartilaginous union (p<0.05). CONCLUSIONS The administration of melatonin maybe beneficial in suppressing the effects of free oxygen radicals and regulating antioxidant enzyme activity in the fracture healing process.