In the course of ageing, biochemical characteristics of mammalian skeletal muscle are shown to become altered mainly within the system of energy metabolism. The enzymatic behaviour of myofibrillar proteins remains virtually unchanged throughout life, but a gradual loss of fibres causes a decrease in musculae strength. Working capacity, however, in terms of prolonged muscular work is considerably reduced with age, because of a less efficient energy metabolism. Enzymes, particularly of the Krebs cycle, show diminished specific activities, and consequently, ATP and phosphocreatine concentrations are lowered. Possible primary and secondary factors responsible for these changes are discussed.