This study investigates the hypothesis that CuZn superoxide dismutase (SOD1) overexpression confers radioresistance to human glioma cells by regulating the late accumulation of reactive oxygen species (ROS) and the G(2)/M-checkpoint pathway. U118-9 human glioma cells (wild type, neo vector control, and stably overexpressing SOD1) were irradiated (0-10 Gy) and assayed for cell survival, cellular ROS levels, cell-cycle-phase distributions, and cyclin B1 expression. SOD1-overexpressing cells were radioresistant compared to wild-type (wt) and neo vector control (neo) cells. Irradiated wt and neo cells showed a significant increase (approximately twofold) in DHE fluorescence beginning at 2 days postirradiation, which remained elevated at 8 days postirradiation. Interestingly, the late accumulation of ROS was suppressed in irradiated SOD1-overexpressing cells. The increase in ROS levels was followed by a decrease in cell growth and viability and an increase in the percentage of cells with sub-G(1) DNA content. SOD1 overexpression enhanced radiation-induced G(2) accumulation within 24 h postirradiation, which was accompanied by a decrease in cyclin B1 mRNA and protein levels. These results support the hypothesis that long after radiation exposure a "metabolic redox response" regulates radiosensitivity of human glioma cells.