Myopathic morphological changes were observed in the calf muscles of rats maintained on a potassium-depleted diet for 30-50 days. Light-microscopic changes were more severe in the superficial gastrocnemius muscle fibres and included pallor, swelling and vacuolation, progressing to necrosis, mononuclear cell infiltration and phagocytosis. Mitochondrial disruption, transverse tubular dilatation, membranous body formation and vacuolation were seen on ultrastructural examination. Both vacuoles and subsarcolemmal membranous bodies were shown to be in continuity with the transverse tubular system. More advanced myopathic changes were those of sarcomere destruction and muscle fibre necrosis. Morphological changes resemble those of human potassium depletion myopathy. Ineffective energy metabolism and focal ischaemia due to impaired vasodilatation are proposed as mechanisms for the myopathy of potassium depletion.