Extensive investigations on the metabolism of animal lenses over the life span indicate various changes that, regarded collectively, suggest a decrease in metabolic energy. A factor analytic interpretation and discussion of these results indicate that enzyme modifications might be one of the fundamental causes of the age-related changes in lens metabolism. Detailed investigations on the kinetic and physicochemical properties of certain enzymes of the carbohydrate breakdown in lenses of different ages verify this assumption. Since the changes observed are found mostly in the lens nucleus protein neosynthesis no longer takes place, posttranslational modifications appear to be responsible.