When young male Wistar rats were fed a diet containing a minute amount of riboflavin, the blood level of each of flavin adenine dinucleotide, flavin mononucleotide, and riboflavin was decreased after 1 week of feeding and remained low after 2 and 3 weeks of feeding. Glutathione reductase activities in the liver and erythrocytes were decreased after 2 weeks of feeding. Then, a significant increase in serum lipid peroxide level was observed one week later. In accordance with the increase in serum lipid peroxides, incipient electron microscopic changes occurred in the lenses of the rats, i.e., swelling of mitochondria in the epithelial cells and the appearance of vacuoles in the fibers. The vacuoles became larger after 6 weeks of feeding on the diet. The administration of linoleic acid hydroperoxide to rats fed the diet containing a minute amount of riboflavin brought about a further increase in serum lipid peroxide level, and the number of vacuoles formed in the lens fibers was increased and their size became larger. These findings support the view that lipid peroxides increased in the blood are the cause of cataractogenesis.