Naphthalene cataract is probably due to peroxide production through naphthoquinone (NQ) redox cycling and/or glutathione conjugation. Both mechanisms yield losses of essential SH-groups in cristallins and are thus probably involved in protein modification finally visible as lens opacity. 1,2-Naphthoquinone produces H2O2 in the presence of either ascorbate, glutathione, NADH or--to a lesser extend--by homogenates of lens protein preparations. In the presence of 1,2-naphthoquinone and the above reductive additions, both, oxygen uptake and H2O2 formation can be observed. Reductive oxygen activation in these systems are diminuated by iodide in a concentration-dependent manner. Since maleimide-treated proteins are less capable to activate oxygen by 1,2-naphthoquinone, a direct oxygen activation by the interactions of 1,2-naphthoquinone with protein-SH is indicated. Catalysis of "diaphorase"-type (dia) enzymes via NADH--dia--1,2-NQ--O2 seems not to operate in hydrogenperoxide production during 1,2-naphthoquinone lens toxicity.