Nitro blue tetrazolium (NBT) staining in normal rat retina was previously found to be affected by inhibition of free radical-related enzyme systems, indicating that NBT might be useful as a marker of free radicals. The aim of the present study was to investigate NBT staining in rat retina in vitamin E deficiency and after blue light exposure, and also to measure hydrogen peroxide (H2O2) production indirectly by measuring catalase activity under these conditions. Vitamin E deficiency resulted in morphological changes in the retina and increased NBT staining in the photoreceptors and in the outer plexiform layer. Light exposure caused increased staining in the inner segments of photoreceptors. The increased staining was not clearly influenced by addition of the free radical scavengers superoxide dismutase and catalase. The catalase activity was not influenced by light exposure, while it was increased in vitamin E-deficient retina compared to controls. The results indicate that reducing systems as measured by NBT were activated in the retina under these conditions. However, to what extent the reductants represent free radicals still has to be established using other methods.