In nuclear cataract and cortical cataract, the enzyme activity pattern of the cortex shows slight alterations, indicating a change in the protein composition. The ATP content of these lenses is high as well as that of fructose. These data indicate a relatively high amount of metabolically active tissue. Subcapsular cataracts do not show significant alterations of the cortical enzyme pattern, but the low content of ATP and fructose indicates a decrease of metabolically active tissue, compared to lenses without opacities in the superficial cortex. In fully opaque lenses, the distortion of the cortical enzyme activity pattern and the low ATP and fructose content indicate a marked decrease of metabolically active tissue. The high glucose content is explained by an increase of the extracellular space, while the unaltered enzyme activity pattern of the epithelium and the high ATP/ADP ratio lead to the assumption that the lens epithelium survives even in early stages of full opacification of the lens.