Lesions were present in the diaphragms of rabbits given high doses of cortisone acetate. Granular muscle fibers appeared to be primarily involved. Increased lipid was noted predominantly in-granular fibers. Glycogen accumulated in both granular and agranular fibers. The degenerating granular fibers showed enlargement of cross-sectional diameter with apparent increase in sarcoplasmic components. Mitochondria with bizarre shapes were a prominent feature in such degenerating fibers. Although these morphologic changes were conspicuous, their signigicance in the pathogenesis of cortisone myopathy could not be assessed.