Abstract： Objective To understand the role of hemne oxygenase-1 (HO-1) in hydrogen peroxide (H2O2) induced apoptosis and mitochondrial trans-membrane potential (MTMP) change in primary alveolar epithelial cell type Ⅱ (AEC Ⅰ ).Methods Primary AEC Ⅰ collected from healthy Sprague Dawley (SD) rats were cultured for 24 hours,then divided into four groups to be treated with:① saline； ② H2O2 (0.5 mmol/L) ; ③ H2O2 + HO-1 (0.2 mmol/L) ； ④ H2O2 + zinc original porphyrin Ⅸ (HO-1 inhibitor,20 μtmol/L ).The morphology of cells in the cultures was examined by fluorescent microscopy 2.5 hours later,and the number of apoptotic cells/the MTMP determined by flow-cytometry 0.5,1.0,1.5,2.0 and 2.5 hours later.Results Large number of cells in with green (early apoptotic) or red (later apoptotic) fluorescence were observed by microscope in cultures treated with H2O2,and H2O2 + HO-1 inhibitor,but such cells were obviously fewer in HO-1 treated cultures.Compared with saline treated cells,H2O2 treated cells had significantly higher apoptosis rate,that increased with time,reaching peak value 2.5 hours into the treatment [0.5 hour:(30.27±0.74)％ vs.(3.76±0.81)％,2.5 hours:(40.46±0.91)％ vs.(22.74± 0.60)％,both P＜0.05],while the rate of MTMP depolarization was significantly lower (0.99± 0.21 vs.1.91±0.16,P＜0.05) in these cells.Compared with H2O2 treated cells,the apoptosis rate in HO-1 treated cells was significantly lower [0.5 hour:(5.99 ± 0.60)％ vs.(30.27±0.74)％,2.5 hours:(22.69± 1.69)％ vs.(40.46±0.91)％,both P＜0.05],and their rate of MTMP depolarization higher (2.02±0.12 vs.0.99±0.21,P＜0.05).Compared with HO-1 treated cells,HO-1 inhibitor treated cells had significantly higher apoptosis rate which reached peak value 2.5 hours into the treatment [0.5 houri (30.73±1.08)％ vs.(5.99±0.60)％,2.5 hours:(41.38±0.57)％ vs.(22.69±1.69)％,both P＜0.05],while rate of MTMP depolarization in these cells was significantly lower (0.98 ± 0.09 vs.2.02 ± 0.12,P ＜ 0.05).Conclusion HO-1 could maintain the integrity of AEC Ⅰ and stabilize their mitochondria membranepotential,protecting the cells from H2O2 induced damage.