The morphological changes of the skeletal muscle cells of the rat experimental myopathy induced by 2, 4-dinitrophenol were examined by scanning electron microscopy in comparison with the ultrastructure of normal muscle cells. Specimens were prepared by the Aldehyde-Osmium-DMSO-Osmium method which permits the three-dimensional demonstration of intracellular structures under SEM. In the specimen prepared by the method, myofibrils having been completely dissolved, intracellular membranous structures such as the sarcoplasmic reticulum, T-tubules and mitochondria were clearly demonstrated in three dimensions. In the experimental mitochondrial myopathy, large accumulations of mitochondria were observed at the subsarcolemmal region. Mitochondria in the perinuclear and intermyofibrillar region showed swelling and occasionally accompanied abnormal concentric cristae. The sarcoplasmic reticulum which showed regular network in normal muscle cells entirely disappeared in the mitochondrial myopathy. Although the mitochondrial changes obtained in this study were almost identical to those previously reported by transmission electron microscopy, the changes in the sarcoplasmic reticulum have not been described in previous works.