This study describes ultrastructural analysis through changes in the cytodifferentiation of myofibrils in cardiomyocytes of cardiac tissue in late embryonic development and early postnatal mouse. The process of progressive differentiation in different cellular spaces of the myofibrils, is a manifestation of the periods studied. In some cells were observed ultrastructure of sarcomeres own, with their respective bands Z, I, A and H, in contrast to other myofibrils that these striations are not observed, possibly because they are in formation process. Myofibrils in embryonic ages are separated from each other by an irregular interfibrillar space; whereas in postnatal ages, have a better organization and many of them are packed in compact confluent masses that fuse to form long fibers branched and arranged following the longitudinal axis of the cell, interspersed with rows of mitochondria. This conformation is related to the elongated shape of cardiomyocytes and increasing the length of the myofibrils; while the presence in the sarcoplasm of large electron translucent areas are reduced with increasing maturity, due to the structural differentiation synthesis, organization of myofibrils and other organelles. These morphological changes in cytoarchitecture are the result of postnatal myocardial adaptation to the demands of work shortly after birth and also are related to the circulatory changes that occur in the neonatal period.