The relationship of the concentration of glutathione (GSH) in lens epithelium to the transport of cations in the lens was studied by decreasing the level of GSH in the epithelium and monitoring subsequent effects in the lens on the distribution of cations, the activity of Na+-K+ ATPase and the uptake and efflux of 86Rb. Oxidation of GSH in cultured rabbit lenses was accomplished by the use of 1 mM tert butyl hydroperoxide (TBHP), a reagent which appears to be suitable for the specific oxidation of GSH in this tissue. The concentration of GSH found in the normal lens epithelium was estimated to be 64 .mu.mol/g weight or nearly 6 times that present in the whole lens. A decrease in the concentration of GSH in lens epithelium of 60% or more led to an increase in hydration; a shift in the distribution of Na+, K+ and Cl-; a decrease in the activity of Na+-K+ ATPase; a decrease in the active transport; and an increase in the passive diffusion of 86Rb. In the TBHP-treated lenses there was a rapid decrease in the production of lactate, possibly as a result of the inhibition of Na+-K+ ATPase, but the effect on the level of lens ATP was delayed and less pronounced. It appeared that the adverse effect on membrane permeability caused by the oxidation of GSH was partially reversed when a high level of GSH returned to the epithelium. The decrease in active transport of 86Rb and the inactivation of Na+-K+ ATPase were not reversed by either regeneration of GSH in the tissue or by treatment with exogenous dithiothreitol and may indicate an irreversible conformational change in the enzyme initiated by the loss of the protective effect of GSH. Apparently a critical level of GSH is required in the lens epithelium for the maintenance of normal cation transport.