This dissertation explores the radiomodulatory affects of the Nrf2/Antioxidant Response Element (Nrf2/ARE) pathway. Exposure to ionizing radiation produces a large amount of reactive oxygen species (ROS) that are a major source of DNA damage. Activation of the Nrf2/ARE pathway increases production of antioxidant enzymes that might be expected to be cytoprotective. However, initial studies with several Nrf2-inducing agents found treatment prior to ionizing radiation failed to protect lymphocytes against radiation-induced apoptosis and several other cell types against cell death in longer term, clonogenic assays.;Nrf2-deficient mouse models were then used to test the impact of the Nrf2/ARE pathway on intrinsic cellular radiosensitivity. Mouse embryo fibroblasts with loss of Nrf2 had decreased expression of heme oxygenase 1 (HO-1) protein expression and glutathione activity, as expected, and after exposure to ionizing radiation dramatically increased ROS formation relative to wild type cells. Although no differences in initial DNA double strand breaks was observed, Nrf2 deficient cells were more radiosensitive under both aerobic and hypoxic conditions. Furthermore, Nrf2-deficient mice were sensitive to whole body irradiation with doses that were sublethal for wild type and heterozygous mice, supporting a radioprotective role of the Nrf2/ARE pathway.;Studies finally focused on the ability of repeated damage to activate the Nrf2/ARE pathway over time, using a gene reporter system. No change was observed 1 to 24 hours after single dose radiation exposure, but by day 5 there was a dose-dependent increase in ARE-reporter activity above 2Gy. This correlated with expression of HO-1 in several cell lines. Similar results were obtained with five daily fractions of I to 4Gy given to cells in vitro and in spleens from irradiated mice. These data suggest that ARE-activation of cytoprotective enzymes is due to persistent oxidative stress after ionizing radiation exposure.;These studies are the first to report on the impact of the Nrf2/ARE pathway on intrinsic cellular radiosensitivity and activation by ionizing radiation.