The concentrations of hydrogen peroxide in the aqueous humor and urine of several animal species and humans have been determined. The determinations are based on peroxide-dependent decarboxylation of I-[14C]-alpha-ketoglutaric acid and measurement of the resulting 14CO2 by quantitating the radioactive disintegration. The levels of H2O2 in most animals varied between 5.0 and 41 microM for aqueous, and 115 and 187 microM for urine. The levels of peroxide in the urine of steer, cat and baboon were lower and fell out of the above range. In the aqueous of humans with cataracts, the levels ranged from 33 to 324 microM, the overall average being 189 +/- 88 microM. The source of such high levels in the aqueous of cataract patients is currently being studied.