In the adult mammalian heart, the cardiomyocytes and thus their terminally anchored myofibrillar bundles are connected by large arrays of closely spaced or even fused adhering junctions (AJs), termed "intercalated disks" (IDs). In recent years, the ID complex has attracted special attention as it has become clear that several human hereditary cardiomyopathies are caused by mutations of genes encoding ID marker proteins, in particular some that are also known as constituents of epithelial desmosomes. Previously, we have shown that in the mature myocardial ID the compositional differences between desmosome-like and adhaerens junctions are, by and large, lost and a composite hybrid structure, the area composita, is formed. We now report results from immunofluorescence and (immuno-)electron microscopic studies of heart formation during mouse embryogenesis and postnatal growth and show that the formation of the IDs with extended area composita structures is a late, primarily postnatal process. While up to birth small distinct desmosomes and AJs are resolved as predominant ID structures, areae compositae of increasing sizes and merged marker protein patterns occupy most of the IDs in the mature heart. Differences in the patterns of ID formation and amalgamation of the two ensembles of junction proteins in time and space are also demonstrated. Together with corresponding observations during rat and human heart development our results indicate that ID topogenesis and area composita formation are also late developmental processes in other mammals. We discuss the importance of the ID and the areae compositae in cardiac functions and, consequently, in cardiomyopathies and possible myocardial regeneration processes.