Lung epithelial cells produce increased reactive oxygen species (ROS) after hypoxia exposure, and they are more susceptible after hypoxia to injury by agents that generate superoxide (O₂^-) (e.g., DMNQ). Cellular GSH and MnSOD both decrease in hypoxic lung epithelial cells, altering the redox state. Since ROS participate in signaling pathways involved in cell death or survival, we tested the hypothesis that mitogen activated protein kinases (MAPK) were involved in a protective response against cellular injury during reoxygenation. Human lung epithelial A549 cells were incubated in hypoxia (<1% O₂ for 24 h) and then reoxygenated by return to air. p38^mapk and MKK3 phosphorylation both decreased after hypoxia. During reoxygenation, cells were incubated with 2, 3-dimethoxy-1, 4-naphthoquinone (DMNQ) (0-50 µM), a redox cycling quinone that produces O₂^-. Hypoxia preexposure significantly increased epithelial cell lysis due to DMNQ. Addition of the p38^mapk inhibitors SB202190 or SB203580 markedly increased cytotoxicity, as did the MEK1/2 inhibitor PD98059 (all 10 µM), suggesting a protective effect of downstream molecules activated by the kinases. Transfection of A549 cells with a dominant active MKK3 plasmid (MKK3[Glu]) partially inhibited cytolysis due to DMNQ, while the inactive MKK3 plasmid (MKK3[Ala]) had less evident protective effects. Stress related signaling pathways in epithelial cells are modulated by hypoxia and confer protection from reoxygenation, since hypoxia and chemical inhibition of p38^mapk and MEK1/2 similarly increase cytolysis due to O₂^-.