Functional states of cardiac contractile apparatus and mitochondria were studied in hereditary cardiomyopathic hamsters (CHF 146) and control golden hamsters using cardiac fibers skinned by two different techniques. The Triton X-100 skinned fibers obtained from diseased animals of 175 to 200 days old, or from control animals, demonstrated the same resting and maximal Ca-activated tensions, the same stiffness, the same rate of tension recovery after quick stretch; the fibers from cardiomyopathic animals differed only by a slightly increased calcium sensitivity. Functional activity of myofibrillar creatine kinase in cardiomyopathy was decreased as indicated by a smaller shift in the pMgATP/rigor tension curve to lower [MgATP] in the presence of phosphocreatine and by a slower rate of the tension recovery after quick stretch in the presence of phosphocreatine and ADP (without ATP). The saponin-skinned fibers allow evaluation of the respiration properties of the total tissue mitochondria. Data obtained in the preparations isolated from diseased animals of two ages (75 to 100 and 175 to 200 days) showed that the ratio of maximal ADP-stimulated respiration rate to the respiration rate in the absence of ADP (an analog of respiration control index) was unchanged in myopathy as compared with age-matched controls. However stimulation of respiration after an addition of creatine at submaximal ADP concentration was observed to be respectively 1.45 times and 3.5 times less in the preparations from younger and older myopathic animals as compared with their respective controls, thus indicating the impairment of functional coupling between mitochondrial creatine kinase reaction and oxidative phosphorylation. These results suggest that hereditary cardiomyopathy is associated with alterations in myocardial creatine kinase system, while myofilaments and mitochondria preserve their basic functional properties.