Abstract： Objective To investigate the effect of ischemic post-conditioning (IPO) on the level of Heme oxygenase-1 (HO-1) in acute ischemia - reperfusion (I/R) injury of lung in order to illuminate its protective mechanism.Methods Forty-eight adult SD rats were randomly divided (random number) into 6groups ( n =8 each):sham operation group ( S group) ; I/R group in which the hilum of left lung was clamped for 45 min followed by 105 min reperfusion; IPO group in which left lung hilum was clamped for 45min and post - conditioned by alternation of 30 s reperfusion with 30 s re-occlusion for three times before perfect perfusion for 102 min; Hemin (HM) + I/R group; ZnPPⅨ (zinc protoporphyrin Ⅸ) + IPO group and HM + S group.Arterial partial pressure of oxygen ( PaO2 ) and malondialdehyde (MDA) content in blood serum were assayed.The left lung was removed for determination of wet/dry (W/D) lung weight ratio and level of HO-1 protein was detected by immunohistochemical technique and pathohistological changes were observed under light microscopic examination. Comparisons among multiple groups were studied by using one-way analysis of variance (ANOVA). Statistical comparisons within groups were analyzed by using paired t -test.Results The level of HO-1 in lung tissue was significantly increased in the I/R group compared with the S group and the HM + S group (P ＜0.01,P ＜0.05).Compared to the I/R group,the IPO and the HM + I/R groups had significant higher level of HO-1 ( P ＜ 0.05,P ＜ 0.01 ).The PaO2 was significantly lower in all experimental groups than that in the S group (90 ± 11 ) mmHg.However,the values of PaO2 in the IPO and the HM + I/R groups were higher than that in the I/R group (P ＜ 0.01 ).In addition to severe lung tissue damage evidenced by pathohistological changes,the lung wet/dry (W/D) weight ratio and MDA level in blood serum were significantly higher in the I/R group than those in the S group (P ＜0.01 ),whereas the lung damage was attenuated either by IPO or by HM pretreatment (P ＜ 0.05,IPO or HM + I/R vs.I/R).Conclusions IPO can attenuate the lung ischemia - reperfusion injury through upregulating the level of HO-1 protein and inhibiting lipid peroxidation injury.