Skeletal muscle cells of young and old humans and CBA/Ca mice were examined electronmicroscopically. The mitochondria in old subjects and mice showed age-related changes. The cristae of mitochondria became irregularly spaced, disrupted and replaced by lamellar, myelin-like structures. Giant mitochondria were often visible. They contain lipofuscin in the myofibrils, too, which was often in close relationship with the damaged mitochondria. Comparing these mitochondrial changes with those demonstrated in the lymphocytes in authors' earlier reports, it can be stated that mitochondrial changes occur more frequently in the skeletal muscle cells than in the lymphocytes.