In vivoexposure of grey squirrels to 40W BLB illumination resulted in alterations in the state of the lens crystallins; mainly in the outer layer of the lens. HPLC revealed an increase of the void volume or crosslinked crystallins and an increase in peptides with molecular weights lower than 20,000 d.In vitroexposure of squirrel lens aqueous extracts to Woods lamp radiation (predominantly 365 nm) led to similar but more exaggerated changes as viewed by high performance liquid chromatography. When viewed by polyacrylamide gel electrophoresis (PAGE), soluble protein crosslinking was also observed. The near-UV absorbing chromophores of low molecular weight present in the lens served as photo-sensitizers that enhanced the protein changes. Sodium azide inhibited the changes, indicating a role for singlet oxygen in the crosslinking.