T o elucidate the metabolic abnormality of musclar dystrophy, 27 kinds of enzyme activity in various organs of control and dystrophic mice were examined. The organs examined included muscle, bone, heart, testis, uterus, spleen, thymus, submaxillary gland, stomach, pancreas, liver, kidney, brain, and lung. The activities of 14 different aminopeptidases, 5 endopeptidases, 4 glycosidases, phosphatase, esterase, and ribonuclease were measured. Most of the enzyme activities were significantly elevated in muscles and bones of dystrophic mice. These organs were similar in their patterns of enzyme abnormality. Among the 14 kinds of aminopeptidase activity studied, the degree of increased activity was greater for the aminopeptidases (AP):Ala-AP, Leu-AP, Met-AP, Phe-AP, Trp-AP, Gly-Pro-Leu-AP. In addition to aminopeptidases, there were significant increases in activities of chymotrypsinlike enzyme, cathepsin C, cathepsin D, several glycosidases and neutral ribonuclease in the muscles of dystrophic mice. Similarly increased enzyme activity was also observed in organs other than muscle and bone. Furthermore, protein content in most organs was higher in dystrophic mice than in those of control mice. These abnormalities were seen in both males and females. The present results suggest that there are extensive abnormalities in the protein metabolism in dystrophic mice. It seems therefore that the therapeutic approach to muscular dystrophy should be studies not only from the well-known abnormality of intramuscular endopeptidases, but from other aspects as well.