Cardiomyocytes of chronic hibernating myocardium are affected by partial to complete loss of sarcomeres, accumulation of glycogen, adaptations in size and shape of mitochondria, reorganisation of nuclear chromatin and depletion of sarcoplasmic reticulum. The nature of these changes, which from a purely morphologic viewpoint are akin to dedifferentiation, needed further clarification at the molecular level. For this purpose we have studied the expression and reorganization of titin, one of the earliest markers of cardiomyocytes differentiation. By use of monoclonal antibodies, recognizing different epitopes distributed over the whole length of the titin molecule, we were able to detect changes in its molecular organization as a result of chronic hibernation. The epitopes of the titin molecule attached to the Z-disc and those present close to the M-line remained detectable at all stages of hibernation, while epitopes at the A-I junction and parts of the myosin anchoring region of the molecule became masked or were lost. A fragmented or punctuated appearance of the titin staining pattern with antibodies to A-I junction related epitopes is found in cells which we consider to represent a more advanced stage of dedifferentiation. Changes in the distribution of the titin molecule or its molecular environment in hibernating myocardium resemble at least in part changes occurring during muscle cell differentiation, although in reversed order.