Objective Oxygen therapy is widely used to improve blood oxygen saturation and reduce tissue hypoxia for patients.However,supplemental O_2 of prolonged high concentration administered to patients may result in oxidative lung injury. Alveolar type ATⅡepithelial cell (ATⅡcell) can function as a progenitor for regeneration after lung injury. Extracellular signal-regulated kinase (ERK) plays a role in cell proliferation, survival and apoptosis, which may have effect on survival or death of ATⅡcells under oxidative stress. In the present study, we investigated the survival and apoptotic responses of primary cultured rat ATⅡcells induced by hydrogen peroxide (H_2O_2) and the regulation mechanism mediated by ERK.  Methods Rat ATⅡcells were isolated by trypsin digestion and differential adherence to plates coated with IgG. Primary cultured rat ATⅡcells were then stimulated by H_2O_2 to simulate ROS attack in vivo. Cells were divided into control group(cells in serum-free culture medium) ,