Objective: To observe the effect of captopril on the tumor necrosis factor a (TNF-alpha) level and arterial blood gases in acute lung injury (ALI) induced by HCL in rats, and to analyze its protective mechanism. Methods: Fifty Wistar rats were selected and randomly divided into three groups, with 20 rats in Group I and II, respectively and 10 animals in Group III. ALI model was constructed by intratracheal injection of diluted hydrochloric. acid (pH=1.25, 1.2 mL/kg). Group I rats received not any treatment after construction of ALI model. Group II rats were treated with captopril (5 mg/kg, i.p.) 5 min after induction of ALI. Group III served as normal control without any treatment. Ninety minutes after construction of ALI model, all the rats were sacrificed. Blood was withdrawn for detection of TNF-alpha level and arterial blood gases index. And lung tissue slices of the three groups were prepared for observation of pathologic histology changes. Results: TNF-alpha level in serum of Group I and II rats was significantly higher than that in Group III (P<0.05), while TNF-alpha level in serum of Group II was significantly lower in Group I (P<0.05). PaCO2 level was significantly higher (P<0.05), while PaO2 was significantly lower (P<0.05) in Group I and if rats than those in Group fir PaCO2 was significantly lower (P<0.05) and PaO2 was significantly higher (P<0.05) in Group II than those in Group I. Histological observation showed diffuse congestion and severe edema of lung tissue, obvious thickening and structure damage of alveolar walls and a large amount of neutrophil infiltration in Group I rats. Group II rats showed mild edema of lung tissue; only a small portion of alveolar walls showed thickening and only a few of neutrophil infiltration could be observed. The degree of injury was remarkably slighter than that of Group I rats. Group III rats showed clear lung tissue structure and normal morphology; alveolar walls were uniform and the margin was smooth and few neutrophil could be observed. Conclusions: Captopril can significantly reduce serum TNF-alpha level, elevate PaO2 and reduce PaCO2 in rats with ALI. It has a protective effect on ALI rats.