The hereditary cardiomyopathy of the Syrian hamster (strain BIO 8262) as a model of human cardiomyopathy was investigated during postnatal development using ultrastructural morphometric analysis. The aim of the study was to establish basic ultrastructural morphometric data of cardiomyocytes of this model during all stages of the disease in comparison to corresponding stages of normal development, and to compare the ultrastructural resistance of normal and myopathic myocardium against acute hypoxic stress. The postnatal development of this cardiomyopathy is characterized by focal myolytic lesions already in the first postnatal days, extended myofibrillic and mitochondrial damage developing after the 1st month, and features of hypertrophy, disarray of myofibrils, thickening, and clumping of z-lines after the 4th month. Morphometric parameters of myopathic cardiomyocytes differing from those of the control are diminished volume density of myofibrils by about 10% of the control values in middle age, slightly increased volume density of sarcoplasmic reticulum and t-tubules in the 1st months, significant increase of them after the 3rd month, and significant alterations of many mitochondrial parameters indicating degenerations (numeric density, volume density, average volume, volume density of cristae, and of destroyed areas, and ratio of surface to volume density of mitochondrial cristae). Acute isobaric hypoxia was applied to myopathic and healthy hamsters of corresponding age (at day 100). Evaluation of ultrastructural morphometric parameters of cardiomyocytes revealed that especially mitochondria were more affected by hypoxia in the myopathic animals. The average mitochondrial volume was significantly more increased in the myopathic condition, the ratio of surface to volume density of cristae significantly diminished. Our ultrastructural - morphometric study has shown, that morphometric parameters in general can be assigned to the stages of cardiomyopathy, and that the ultrastructural differences are best expressed about the 4th month.