Immunocytochemistry using antibodies against myofibrillar proteins such as heart-C-protein and myomesin but also the fluorescence of rhodamine conjugated phalloidin as marker for F-actin structures was employed to study the redifferentiation of cultured adult rat cardiomyocytes. It was demonstrated that freshly isolated rod shaped cells in culture round up before they attach to the substratum, while myofibrillar structures of the cells degenerate. As soon as they attached to the substratum and gradually flattened out, small cross-striated myofibrils reappeared in the central part of the cells. F-actin, in freshly isolated cells present only in the I-band of the myofibrils, extended throughout the cells into the processes in the form of filament cables. In later stages the myofibrillar distribution varied. Smaller myocytes were frequently filled with myofibrils whereas in larger cells with many processes the myofibrils were found either arranged in perinuclear regions or showed a mosaic distribution. Myofibrils appear to be directly aligned with bundles of actin stress fibers, thus further supporting the suggestion that the actin filaments might serve as a scaffold for myofibril formation.