Several enzymatic and nonenzymatic reactions play important roles in the physiologic neutralization of hydrogen peroxide (H2O2) in the anterior segment of the eye. The nonenzymatic reactions are particularly important in the aqueous humor, where enzymes are normally absent and high levels of ascorbate are present. One of ascorbate's presumed functions is to protect the lens and retina from the damaging effects of ultraviolet radiation. It also appears to act as an antioxidant for the removal of H2O2. Although H2O2 is frequently a product of antioxidant reactions, the low oxygen tension of the aqueous humor and the absence of trace elements apparently account for the relatively low concentrations of H2O2. This property of aqueous humor is important because high concentrations of H2O2 are toxic to both the lens and the cornea. This damage is exacerbated by the removal of glucose or by inhibition of glutathione reductase--an indication of the importance of the glutathione redox cycle in protecting against endothelial damage induced by H2O2. Catalase also protects the tissues bordering the anterior chamber from H2O2-induced damage. Decreasing catalase activity by treatment with 3-aminotriazole increases the time required for H2O2 clearance from the anterior chamber, thereby allowing more time for H2O2 to produce damage. A decline of catalase activity with age has been observed in the iris and corneal endothelium of rabbits.