In order to clarify cross-sectional morphology of tetralogy of Fallot in the fetus, 202 rat fetuses with tetralogy of Fallot induced by maternal administration of bis-diamine were studied using cesarean section, the whole body freezing method and by photographing cross sections of the fetal heart on a freezing microtome. Some characteristic features such as large subaortic ventricular septal defects, aortic overriding over the ventricular septum, aortomitral fibrous continuity, and narrow infundibulum and pulmonary valve were found. Moreover, three subtypes were noticed. In classic tetralogy of Fallot with stenotic infundibulum, the size of the pulmonary arteries was smaller in proportion to the infundibular stenosis. In the second type with absent infundibular septum and severe valvular pulmonary stenosis, the pulmonary arteries were most hypoplastic. In the third type with absent pulmonary valve, the pulmonary arteries were dilated forming aneurysms. In the last type only, ventricles were dilated and grossly hypertrophic.