When young mice were exposed to 40-watt black light lamps for 12 h a day, microanatomical and biochemical changes were observed in their lenses after 35 weeks. From this time on, many undifferentiat-Mice ed cells with pyknotic nuclei were found in the lens cortex. Accumulation of soluble crystalline was depressed, especially γ-crystallin. High levels of insoluble protein were found in lenses of UV-irradiated mice. High molecular weight aggregates increased in this insoluble phase. From 60 weeks on, cortical opacities were seen. Thus, cataracts have now been shown to result from biochemical and anatomical changes in lenses of mice exposed to low levels of near UV light for extended time periods.