Objective: To study the change and role of hemeoxygenase-1 (HO-1) in injured lungs following limbischemia/reperfusion in rats.Methods: A total of 96 healthy male Sprague-Dawley rats, weighing 250-300 g, were used in this study. Hind limb ischemia was made on 40 rats through clamping the infrarenal aorta for 2 hours with a microvascular clip, then limb reperfusion for 0, 4, 8,16 and 24 hours(n =8 in each time point)was performed, respectively. Other 8 rats undergoing full surgical operation including isolation of the infrarenal aorta without occlusion were taken as the sham operation group. Lung tissues were obtained from the 48animals and North blotting and West blotting were employed to measure the changes of HO-1 mRNA and protein expression, respectively. Immunohistochemistry technique was used to determine the cell types responsible for HO-1 expression after limb ischemia/reperfusion. Then hind limb ischemia was made on other 12 rats through clamping the infrarenal aorta for 2 hours with a microvascular clip, among whom, 6 rats were given zinc protoporphyrin (ZnPP), an inhibitor of HO. Then limb reperfusion for 16 hours was performed on all the 12 rats.And other 12 rats underwent full surgical operation including isolation of the infrarenal aorta without occlusion,among whom, 6 rats were then given ZnPP. Then lung tissues were obtained from the 24 animals and lung injury markers, lung histology, polymorphonuclear leukocyte (PMN) count and malondialdehyde (MDA) content were detected, respectively. HO activity was determined through measuring the carboxyhemoglobin (COHb) level in artery blood with a CO-oximeter after limb ischemia/reperfusion.And the animal mortality was observed on the other 24rats.Results: North blotting analysis showed that HO-1mRNA increased significantly at 4 hours after reperfusion,peaked at 16 hours, and began to decrease at 24 hours. In contrast, no positive signal was observed in the sham and simple ischemia animals. Increased HO-1 mRNA levels were accompanied by similar increases in HO-1 protein.Lung PMNs and MDA content increased significantly at 4,8, 16 and 24 hours after reperfusion, compared with the sham controls (P ＜ 0.001 ), while they decreased in rats with reperfusion for 16 hours when compared with rats with reperfusion for 4 hours (P ＜ 0. 001 ). Immunohistochemical studies showed that HO-1 was expressed in a variety of cell types, including the airway epithelia, alveolar macrophages and vascular smooth muscular cells. The blood COHb level and animal mortality increased significantly after limb ischemia/reperfusion compared with the sham controls (P＜ 0.001 ). ZnPP administrated to the ischemia/reperfusion animals led to a decrease in the COHb level and an increase in lung PMN number, MDA content and animal mortality (P＜0.001 compared with ischemia/reperfusion group),and the lung injury was aggravated.Conclusions: Limb ischemia/reperfusion up-regulates pulmonary HO-1 expression, which serves as a compensatory protective response to the ischemia/reperfusion-induced lung injury in rats.