We isolated and identified a stress protein that is up-regulated in response to hypoxia in primary-cultured glial cells. Protein-disulfide isomerase (PDI) was up-regulated not only by hypoxia in glia in vitro, but also by transient forebrain ischemia in rats in vivo. To determine whether newly synthesized PDI is involved in tolerance to ischemic stress, we carried out two procedures to induce PDI gene expression in human neuroblastoma SK-N-MC cells, as well as intrahippocampal injection following electroporation of an expression vector capable of overexpressing PDI in rats. Overexpression of this gene resulted in attenuation of the loss of cell viability induced by hypoxia in neuroblastoma SK-N-MC cells and a reduction in the number of DNA-fragmented cells in the CA1 area of the hippocampus in brain ischemic rats, respectively. These findings suggest that up-regulated PDI may play a critical role in resistance to ischemic damage, and that the elevation of levels of this protein in the brain may have beneficial effects against brain stroke.