Objectives: Little is known about the causal factors which induce the typical structural changes accompanying cardiomyocyte dedifferentiation in vivo such as in chronic hibernating myocardium. For identifying important factors involved in cardiomyocyte dedifferentiation, as seen in chronic hibernation, an in vitro model mimicking those morphological changes, would be extremely helpful. Methods: Adult rabbit cardiomyocytes were co-cultured with cardiac fibroblasts. The typical changes induced by this culturing paradigm were investigated using morphometry, electron microscopy and immunocytochemical analysis of several structural proteins, which were used as dedifferentiation markers, i.e., titin, desmin, cardiotin and alpha -smooth muscle actin. Results: Close apposition of fibroblasts with adult rabbit cardiomyocytes induced hibernation-like dedifferentiation, similar to the typical changes seen in chronic hibernation in vivo. Both changes in ultrastructure and in the protein expression pattern of dedifferentiation markers as seen in chronic hibernating myocardium were seen in the co-cultured cardiomyocytes. Conclusion: Hibernation-like changes can be induced by co-culturing adult rabbit cardiomyocytes with fibroblasts. This cellular model can be a valuable tool in identifying and characterizing the pathways involved in the dedifferentiation phenotype in vivo, and already suggests that many of the structural changes accompanying dedifferentiation are not per se dependent on a decreased oxygen availability.