Our previous studies have shown that exposure of cultured rabbit lenses to physiological levels of hydrogen peroxide, following inhibition of the glutathione redox cycle, leads to the formation of distinct vacuoles in the anterior region of the lens at the germinative zone between the epithelium and lens fibers. In the present study the ultrastructure of H2O2-induced membrane damage in the intact lens and in cultured lens epithelial cells was examined by scanning and transmission electron microscopy (SEM and TEM), following the inhibition of glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Lenses treated with BCNU/H2O2 exhibited swollen epithelial cells which were observed only above the peroxide-induced vacuoles. The apical surface of the swollen cells had membrane blebs which protruded into the underlying vacuolar space. The appearance of the blebs coincided with a change in the organization of the layer of microfilaments which is normally associated with the apical surface of the cell. Cultured lens epithelial cells treated with BCNU/H2O2 showed membrane blebs which increased in size and number with the duration of exposure. Initially, the blebs were seen only on certain regions of the cell surface with other regions appearing normal. TEM revealed a disorganization of microfilaments in the BCNU/H2O2 treated cells. Neither BCNU nor H2O2 alone affected the morphology of intact lenses or of cultured lens epithelial cells. In culture, isolated lens epithelial cells exposed to BCNU/H2O2 were more susceptible to damage than contiguous cells. While the exact mechanism by which H2O2-induced damage leads to bleb formation on the cell surface is not known, the inability of the cells to detoxify H2O2 due to the inhibition of glutathione reductase results in the disturbance of membrane cytoskeleton and a focal weakening of the cell surface. These results indicate a correlation between the active glutathione redox cycle in lens epithelium and maintenance of normal cytoskeletal protein organization.