In 19 human fetal and newborn hearts with atrioventricular septal defect (AVSD), not associated with other anomalies, the two ventricles were studied morphometrically. A total of 17 different parameters were studied: nine in the right ventricle and eight in the left. In the right ventricle we analyzed ventricular wall thickness, length of right ventricular inflow and outflow tracts, and volume of right ventricular inflow and outflow tracts. The data for these parameters were compared with the patterns of normality published previously, and the volumetric data were compared with patterns of normality published previously by us. The ventricular inflow tract was shorter than the outflow tract, the difference being especially significant in the left ventricle. The length of the diaphragmatic wall of the heart in both the right and left ventricle was equal to the sum of the length of the inflow tract and the thickness of the ventricular wall at the apex.