The guinea pig is a nest-quitter or nidifugous mammal which develops almost to the weaning stage before birth. Myocardial cells of the bilateral atria and ventricles of guinea pigs were examined for comparison with the rat by an ultrastructural quantitative method during the pre- and postnatal developmental periods, at copulation ages of 50, 57, 64 (term), 73 and 80 days. Throughout the developmental stages, no clear differences between the right and left sides were found in the characteristics of the myocardial cells. At the stage of day 57, T-tubules were first recognized in the ventricular muscle cells, and at the same time the fiber thickness of the ventricular muscle cells (8–9 µm) became greater than that of the atrial cells (6–7 µm). At this stage, the fraction of mitochondria was also found to differ between the atrial and ventricular muscle cells. The general cellular features appeared to change more greatly during the last prenatal week from copulation day 57 to day 64 (term) than during the first postnatal week. Thus, the last prenatal week in the ontogeny of the guinea pig may be interpreted as corresponding to the postnatal period from the 10th to the 20th day in the ontogeny of th