Intracameral hydrogen peroxide (H2O2) is cleared at a faster rate in young (t 1/2, 93 seconds) than in adult (t 1/2, 109 seconds) rabbits. Extrapolated zero time concentrations of H2O2were 3.3 mM in adults and 3.2 mM in young. The more rapid disappearance of H2O2correlated with greater catalase levels in iris (35%) and corneal endothelium (50%) in young as compared to adult animals. Catalase levels have been found to be reduced in ocular tissues with 3-amino–1H-1,2,4–triazole (3AT) in a dose-related manner up to 6 ml/kg of an intravenous 3M solution. Iris and ciliary processes showed a linear reduction with dose, while corneal endothelium, liver and lung reached near maximal decreases in catalase activity at 2, 4, and 6 ml/kg, respectively. 3AT caused a significant dose-dependent extension of the rate of clearance of H2O2from the anterior chamber, that was directly related to catalase loss. The t 1/2 for H2O2disappearance in adult animals increased from 109 seconds with no 3AT, to 147 seconds after 2 ml/kg 3M 3AT, to 161 seconds after 4 ml/kg 3M 3AT and 184 seconds after 6 ml/kg 3M 3AT. Corneal endothelial oxidized glutathione levels were transiently increased after intracameral hydrogen peroxide. Considering the sum total of all tissues of the anterior segment, specific incremental decreases of catalase generated by intravenous 3AT caused the t 1/2 of H2O2clearance from the anterior chamber to become longer, while the reducing power of anterior segment tissues excluding lens epithelium is related clearly to the systemic dose of 3AT. Protection of anterior segment tissues from oxidants depends on the interplay between the concentration of the oxidant, the availability of reduced glutathione and the activity of catalase.