Cardiac structures were measured to determine the sequence of alterations during the development of spontaneous and furazolidone-induced cardiomyopathy in turkey poults 20 to 31 days after hatching. Samples of tissue from the free walls of the ventricles were removed and processed for electron microscopy. Volumetric density of cardiac structures was determined by the point-counting technique. The results indicate that the sequence of myocardial alterations is similar in both spontaneous and furazolidone-induced cardiomyopathy, but that these changes are more pronounced in the latter condition. Major changes include an early diminution in myofibrillar density, accompanied by a small reduction in mitochondrial density. This is followed by dilatation of the ventricular lumina and a decrease in the volume fraction of Z-bands. The increase in mass of the free walls of the ventricles in affected poults is due primarily to an increase in extracellular components and sarcoplasm devoid of organelles. The results suggest that the development of this myopathy is initiated by damage to the myofibrils of the cardiocytes.