After rats had been fed a low-potassium diet for 4 to 8 weeks, skeletal muscle showed ultrastructural changes involving membranous organelles. Mitochondria were often swollen, condensed, or disintegrated. The transverse tubules were disoriented, focally dilated, and tortuous. The sarcoplasmic reticulum showed various degrees of dilatation. Vacuoles of different sizes occurred frequently. Whirls of membranes were closely associated with any of the membranous organelles, especially near an orifice of a transverse tubule. Supplementation of potassium reversed these changes. These findings are very similar to those in patients with hypokalemic periodic paralysis. The vacuolar myopathy in these patients may be secondary to the electrolyte alteration in skeletal muscles, and the chronic weakness of some patients may be due to excitation-contraction uncoupling as a result of the involvement of sarcotubular systems.