The aim of this study was to investigate the antioxidant roles of different doses of melatonin (5 and 10 mg kg ^-1 ) against γ-irradiation-caused oxidative damage in liver tissue after total body irradiation (TBI) with a single dose of 6.0 Gy. Fifty adult rats were divided into 5 equal groups, 10 rats each. Groups I and II were injected with 5 and 10 mg kg ^-1 of melatonin, and group III was injected with an isotonic NaCl solution. Group IV was injected with only 5 mg kg ^-1 of melatonin. Group V was reserved as a sham control. Following a 30-min-period, 6.0 Gy TBI was given to groups 1, 2 and 3 in a single fraction. The liver malondialdehyde (MDA) levels, super oxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities were measured in all groups. TBI resulted in a significant increase in the liver tissue MDA levels and a decrease of SOD and GSH-Px activities. The results demonstrated that the liver tissue MDA levels in irradiated rats that were pretreated with melatonin (5 or 10 mg kg ^-1 ) were significantly decreased, while the SOD and GSH-Px activities were significantly increased. Decreasing the MDA levels by melatonin was dose dependent, but the liver tissue SOD and GSH activities were not. The data obtained in this study suggest that melatonin administration prior to irradiation may prevent liver damage by irradiation.

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