Abstract: Objective: To study the changes of neutrophil elastase（NE）of blood serum,blood gas analysis in endotoxin（ET）-induces acute lung injury（ALI） in rabbits by intravenous injection,and to observe the protective effects of Chloroquine in order to reveal the mechanism of ALI and to find out effective therapeutic medicine for ALI in the clinic.Methods: 24 Rabbits were randomly assigned to three groups: group A（control group）,group B（ET group）,and group C（ET and chloroquine group）.ALI was induced by intravascular injection of ET（600 μg/kg）.The rabbits of group C were injected by chloroquine after ET injection.The rabbits of group A were injected by normal sodium.The contents of NE,malondialdehyde（MDA）,superoxide dismutase（SOD） in blood and blood gas analysis were measured before injection and after injection 0.5 h,1 h,2 h,4 h in every group.The results were analyzed by SPSS 13.0.Results: NE and MDA are higher in group B than that in group A.PaO₂/FIO₂,SOD in group B are lower than that in group A.The results of group C are similar with group B.But the extent of change in group C is lower than that in group B.The differences with each group are significant.Conclusion: The action of NE enhancement is one of the mechanisms of ALI,Chloroquine can depress the action of NE and it has protective effect to endotoxin-induced ALI.