The antioxidant effect of reduced plastoquinone was studied in chloroplast membranes. Isolated spinach thylakoid membranes were subjected to strong illumination followed by analysis of pigment bleaching and lipid peroxidation. The plastoquinone pool was kept in the reduced or oxidized state during the light stress by the addition of the electron transport inhibitors 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and o-phenanthroline, respectively. In the absence of inhibitors there occurred a bleaching of carotenoids and chlorophyll a, while chlorophyll b was unchanged. Formation of thiobarbituric acid reactive substances, used as a measure of lipid peroxidation, was negligible during the first hour of strong illumination, but during the second hour there was a marked increase in the rate of lipid peroxidation. Reduction of the plastoquinone pool resulted in a virtually complete inhibition of lipid peroxidation and pigment bleaching. In contrast, conditions of an oxidized plastoquinone pool markedly enhanced lipid peroxidation and pigment bleaching. It is argued that the reduced form of plastoquinone can act as a scavenger of toxic oxygen species generated in the thylakoid membranes during strong illumination.