Abstract： Objective To investigate the antioxidant effect of melatonin (MT) in the rats with phosgene-induced lung injury and its possible mechanism.Methods Fifty male SD rats were equally randomized into phosgene exposure group,air control group,MT treatment group,dexamethasone (DX)treatment group,and negative control group.All groups except the air control group were exposed to 8.33 mg/L phosgene for 5 min,and the MT treatment group,DX treatment group,and negative control group were injected with MT (10 mg/kg),DX (2.5 mg/kg),and 1％ ethanol saline (1 ml/kg),respectively,via the caudal vein 1 hour after exposure.The rats were sacrificed 6 h later.Then,the wet/dry ratio of the lung,the total protein content and neutrophil count in bronchoalveolar lavage fluid (BALF),and the malonaldehyde (MDA) content and superoxide dismutase (SOD) and myeloperoxidase (MPO) activities in lung homogenate were measured;pathological observation was made on the lung tissue under an optical microscope; the protein expression of inducible nitric oxide synthase (iNOS) and NF-kB in the lung tissue was measured by Western blot.Results Compared with the air control group,the phosgene exposure group showed significantly increased wet/dry ratio of the lung and total protein content and neutrophil count in BALF (P＜0.01) as well as significantly increased MDA content and MPO activity in the lung tissue (P＜0.05).Compared with the phosgene exposure group,the MT treatment group showed significantly decreased MDA content and MPO activity and significantly increased SOD activity (P＜0.01),and the MT treatment group and DX treatment group showed significantly decreased protein expression of iNOS and NF-kB (P＜0.01).Conclusion MT has protective effect in phosgene-induced lung injury,and its protective mechanism may be associated with scavenging free radicals and inhibiting expression of iNOS and NF-kB.