OBJECTIVE To determine the protective mechanism of preconditioning to the lung injury induced by ischemia-reperfusion. METHODS Twelve pigs were randomly divided into 2 groups: control group ( Group C) and ischemic preconditioning group ( Group IP). The concentration of superoxide distrautase (SOD) and malondialdehyde (MDA) in circuit were checked before and after the perfusion to reflect the lipid peroxidation in the lungs. Left lung biopsies were performed immediately after the perfusion and 1 hour postperfusion for histologic examination. The ICAM-1 expression was assessed by immunohistochemical analysis with Envision method and the mRNA expression of ICAM-1 was analyzed by RT-PCR. RESULTS SOD in Group IP was much higher than that in Group C (P < 0.01 ). MDA in Group IP was much lower than that in Group C ( P < 0.01 ). The lung histologic examination showed that Group C was significantly more serious than Group IP in pulmonary edema, inflammatory cell infiltration, mild focal hemorrhage, and alveolar disruption. The expression of ICAM-1 of lung tissue obviously decreased in Group IP than that in Group C (P <0.01 ). The expression of ICAM-1 mRNA of lung tissue was significantly lower in Group IP than that in Group C (P < 0.01 ). CONCLUSION Lung ischemic preconditioning can reduce the lung injury. The mechanisms of the protective effects of the IP may be related to the increase of SOD and the decrease of MDA. The preconditioning down-regulated the ICAM-1 expression is one of the mechanisms in reducing the lung injury.