Lenses from normal rabbits, mice, rats, cattle, guinea pigs, lambs, chicken, cats, baboons, blue acara (fish) and dogs were examined for the presence of H2O2. No previous reports exist on the presence and levels of H2O2in normal eye lenses. Freshly isolated lenses of these animals were extracted with trichloroacetic acid and the extract neutralized with Tris. H2O2was assayed in these extracts by reacting them with l-14C-alpha-ketoglutarate and measuring the14CO2produced by peroxide-dependent decarboxylation. Peroxide of the order of 10-−4M was detected in most of the lenses except in baboons wherein it exists between 10-−4and 10-−5M. Culture experiments with rat lenses demonstrated that GSH may make a major contribution to the formation of H2O2in the intact lensin vivo.