Overdistention of lung tissue during mechanical ventilation may be one of the factors that initiates ventilator-induced lung injury (VILI). We hypothesized that cyclic mechanical stretch (CMS) of the lung epithelium is involved in the early events of VILI through the production of reactive oxygen species (ROS). Cultures of an immortalized human airway epithelial cell line (16HBE), a human alveolar type II cell line (A549), and primary cultures of rat alveolar type II cells were cyclically stretched, and the production of superoxide (O₂⁻) was measured by dihydroethidium fluorescence. CMS stimulated increased production of O₂⁻ after 2 h in each type of cell. 16HBE cells exhibited no significant stimulation of ROS before 2 h of CMS (20% strain, 30 cycles/min), and ROS production returned to control levels after 24 h. Oxidation of glutathione (GSH), a cellular antioxidant, increased with CMS as measured by a decrease in the ratio of the reduced GSH level to the oxidized GSH level. Strain levels of 10% did not increase O₂⁻ production in 16HBE cells, whereas 15, 20, and 30% significantly increased generation of O₂⁻. Rotenone, a mitochondrial complex I inhibitor, partially abrogated the stretch-induced generation of O₂⁻ after 2 h CMS in 16HBE cells. NADPH oxidase activity was increased after 2 h of CMS, contributing to the production of O₂⁻. Increased ROS production in lung epithelial cells in response to elevated stretch may contribute to the onset of VILI.