This study was undertaken to evaluate the protective effects of melatonin against cyclophosphamide (CP)-induced oxidative lung toxicity in mice. Mice were pre-treated with various doses of melatonin for 7 consecutive days and were then injected with CP (200 mg/kg b. w.) 1 h after last melatonin injection. After 24 h, the mice were euthanized and their lungs were immediately harvested. Several biomarkers associated with oxidative stress in lung homogenates, such as thiobarbituric acid reactive substances (TBARs) and reduced glutathione (GSH) levels and the activity of superoxide dismutase (SOD) and catalase (CAT) were measured spectrophotometrically. A single dose of CP markedly altered the levels of these oxidative stress biomarkers in lung homogenates. However, increased lipid peroxidation, measured as TBARs, was significantly inhibited in the lung tissues of the melatonin-pretreated mice compared to the CP alone-injected group. In addition, pretreatment with melatonin also significantly restored GSH levels and SOD and CAT activities. Melatonin also effectively protected animals from CP-induced histological abnormalities in lung tissue. In conclusion, the increase in oxidative stress markers and concomitant adaptations by the antioxidant defense system indicates that oxidative stress plays an important role in CP-induced damage to the lung. Moreover, melatonin is a potent natural antioxidant that helps prevent CP-induced oxidative toxicity in mouse lung tissues. Thus, because melatonin is regarded to be a safe pineal secretory product, it may be used concomitantly as a supplement to reduce lung damage in patients undergoing chemotherapy.