In this investigation, the ultrastructural features of the nutritional cardiomyopathy of protein-calorie-malnourished rats were examined. Protein-calorie malnutrition was induced in young rats by feeding them a low-protein diet (4% protein) for 6 weeks. Control animals were fed a high-protein diet (16% protein). The deficient rats showed severe restriction of body-weight gain, fatty liver and hypoproteinaemia. The results of the present study clearly demonstrated that the experimentally induced protein-calorie malnutrition brings about striking morphological changes in the heart of the rat. On light microscopy hyalinization an vacuolization of muscle fibres, loss of cross striations and myofibrils, small foci of necrosis, interstitial fibrosis and mononuclear-cell infiltration could be detected. The ultrastructural lesions were characterized by myofibrillar degeneration, contraction-band formation, dilatation of sarcoplasmic reticulum, mitochondrial swelling, dehiscence of intercalated discs, and widened interstitial spaces, especially around vessels, due to oedema fluid and cellular infiltration by mononuclear cells an activated fibroblasts with collagen fibres and microfibrils. In addition, an increase in relative heart weight was also observed. The potential role of catecholamines in the pathogenesis of this cardiomyopathy is discussed.