Alterations in the activities of two acid hydrolases, cathepsin and acid deoxyribonuclease, and in noncollagen protein nitrogen and deoxyribonucleic acid content of breast muscle of chickens with hereditary muscular dystrophy have been investigated from the 14-day embryo to 1-year-old chickens. In general, dystrophic muscle has a lower noncollagen protein nitrogen content, and higher deoxyribonucleic acid content, deoxyribonuclease, and cathepsin activity than does control muscle. The relative order of appearance of these changes is deoxyribonucleic acid content, deoxyribonuclease activity, noncollagen protein nitrogen content, cathepsin activity. Cathepsin and deoxyribonuclease values for heterozygous and homozygous controls are essentially the same. Noncollagen protein nitrogen and deoxyribonucleic acid content of heterozygous control muscle are generally similar to those of the homozygous, although by 1 to 3 months they tend to approach dystrophic levels.