We studied with quantitative methods the myocardium of 32 specimens of rats divided into three age groups: embryos, fetuses and neonates. Days of gestation were counted from the day following an overnight mating that was considered day 1 of gestation and only one animal per litter was used. The hearts were fixed in Bouin’s fixative, sectioned and stained by routine methods. Stereological determinations were made on ventricular myocardium: (1) volume density of the myocytes [Vv(myocyte)] and interstitium [Vv(interstitium)], and (2) numerical density of the myocytes [Nv(myocyte) 1/mm3] calculated from fifteen optical dissector pairs per specimen. The total number of cardiac myocytes [N(myo-cyte)] was calculated as the product of Nv(myocyte) and the cardiac volume. The Nv(myocyte) increased from embryos to neonates, differences between embryos and fetuses and between embryos and neonates were statistically significant. The Vv(myocyte) increased from embryos to neonates (from 80.0 to 94.0%). During this period the Vv(interstitium) decreased from 21.0 to 5.5%. Differences of the Vv(myocyte) and Vv(interstitium) were significant comparing embryos with neonates and comparing fetuses with neonates. The N(myo-cyte) is roughly (mean ± standard error of the mean) 9,297 ± 487 in fetuses and 38,438 ± 612 in neonates. This represents an increase of about 3.1 times from fetuses to neonates while the cardiac weight increased about 2.2 times in the same period. Coefficients of error for the Nv(myocyte) estimates averaged about 7.3%, for the Vv(myocyte) estimates averaged about 1.8%, and for the Vv(inter-stitium) estimates averaged about 9.1%. These results suggest a high mitotic activity in the rat myocardium during prenatal life and after birth.