The enzymes of adenine nucleotide metabolism in developing skeletal muscle.

1. During late foetal and early post-natal development of rabbit skeletal muscle the total protein increased more rapidly than the non-protein nitrogen content per g. wet wt. 2. AMP-deaminase activity of rabbit leg muscles increased rapidly over the period 5-15 days after birth. In diaphragm muscle from the same animal the rapid increase to the adult enzymic activity took place at about the time of birth. 3. The rapid increase in AMP-deaminase activity of leg muscle occurred earlier in animals born relatively mature, such as the chick and guinea pig, than in animals less well developed at birth, such as the rabbit and rat. 4. The pattern of enzymic activity shown by AMP deaminase during development in diaphragm, leg and cardiac muscles in a given species was closely paralleled by those of adenylate kinase and creatine phosphokinase. 5. When young rabbits were encouraged to become active at an earlier stage than is normal, the rise in creatine-phosphokinase activity occurred at an earlier age than in the control animals. 6. The results suggest that the activity pattern of the muscle is an important factor in determining the time at which the activities of the enzymes of special significance for muscle rise sharply to the adult values. 7. Development in rabbit leg muscle also involved an increase in aldolase activity. The pattern of change was similar to that obtained with other enzymes studied.