During the embryonic development the heart is greatly dependent on the myocyte proliferation, which continues until the neonate. The myocardium regeneration during this period varies directly with the potential of cell proliferation. It is still debatable the proliferative and hypertrophic capacities of the myocardium from embryonic to perinatal period. Five non-consanguineous rat fetuses at 18.5 and 20.5 days of gestation were utilized and fragments of the left ventricular wall were obtained, fixed and processed for routine electron microscopy. Stereological estimation made in semithin sections were obtained using 10 random microscopical fields in each heart with M-42 Test-System mounted in a Nikon CFW eyepiece (10X) and with immersion objective (100X). Qualitative electron microscopic study was performed. The results show the means of volumetric density values: a) Myocytes (including nuclei) = 69.3%; b) The Myocyte Nuclei alone = 6.5%; c) Connective Tissue (including vessels) = 30.7%. The Myocyte Nuclei has a mean of surface density equal to 0.073 µm²/µm³. The mean of numerical density of nuclei was obtained utilizing Disector's Method and the value found was 2357160 /mm³. The diameter of myocyte nuclei, considering like a sphere was 3.7 µm³. The EM presented myocytes well fixed with well-developed sarcomers disposed in an irregular form in the myocyte cytoplasm. The cardiac interstitium showed fibroblasts with characteristics of a great proteic synthesis. The volume of cardiomyocyte nuclei is smaller than that found in the humam myocardium during this period. There are differences in the numerical density of nuclei comparing rat and human. This article suggest binucleate cardiomyocyte during fetal period in rat as greater than was found in previous works