The changes of myoglobin localization in the skeletal muscle cells in Duchenne muscular dystrophy (DMD), myotonic dystrophy (MyD), and amyotrophic lateral sclerosis (ALS) were studied by immunohistochemistry and immunoelectron microscopy. In normal skeletal muscle cells, myoglobin was found in I band, Z line, mitochondrial outer membrane and inner membrane structures of sarcoplasmic reticulum and T tube. In contrast, the myoglobin staining of the I bands in degenerative muscle-cells of DMD and MyD was found rather diminished, and A bands, intraluminal spaces of the inner membrane system and intermyofibrillar spaces were myoglobin positive. Moreover, the I band of a myofibril which slipped out of other normally arranged myofibrils showed no myoglobin staining, but the distended intermyofibrillar spaces adjacent to the slipped myofibril showed a definite staining. In addition, there were dilated sarcoplasmic reticula showing the staining in their lumina, but the I bands neighboring them revealed a diminished staining. In DMD muscle, no staining was found in opaque fibers and some small sized fibers. In ALS muscle, myoglobin was usually positive in the I bands, but the small angulated fibers showed a variable staining. In the target fibers, central zone was not stained but intermediate zone showed altered myoglobin localization. These data indicate that (1) myoglobin localization in the muscle cells varies depending on the sorts of diseases and the grades of muscle cell degeneration, (2) myoglobin in dystrophic muscles fluxes from the muscle cells to extracellular spaces through the dilated sarcoplasmic reticula, T tubes and intermyofibrillar spaces, and (3) in DMD muscle, myoglobin also fluxes directly through the deteriorated plasma membrane in opaque fibers or through the plasma membrane altered due to dystrophin deficiency.