Investigation of lens epithelial cells indicates that under normal conditions, essentially all of the detectable cellular glutathione is in a reduced state. However, exposure to levels of H2O2 in the range found in the aqueous fluid of cataract patients causes rapid, very large changes in the glutathione redox ratios. Immediately following short-term exposure to 0.15-0.2 mM H2O2, reduced glutathione drops to 19% of its normal level and the remainder of the total glutathione is found in the oxidized form. Within the next few minutes, the redox ratio returns to normal. However, total glutathione levels remain approximately 20% below normal even one hour after exposure to H2O2. With exposure to a higher concentration of H2O2, a greater loss of glutathione is observed. The results suggest that the glutathione redox ratios change dramatically as a result of oxidative insult but quickly return to normal when the oxidative insult is removed. The formation of mixed glutathione-protein disulfide was also observed but only after long-term (1 hour) exposure to a high level (0.6 mM) of H2O2.