The early appearance and relatively large size of the embryonic heart suggest that cardiac function is critical to early development. Previous studies had shown that an index derived from curvature and thickness of the ventricular wall provides an estimate of the pressure generating capacity of the myocardium. To obtain an estimate of the functional capability of the embryonic ventricle, images of serial histologic sections of eight normal human embryos, ranging from stages 9-23, from the Carnegie Embryological Collection were chosen for study. The contours of ventricular components were digitized and entered into a computer, and three-dimensional (3-D) reconstructions were created. Volumes of the components of the ventricles were determined, including the compact or outer portion of the ventricular wall, the cardiac jelly, the overall volume containing trabeculated myocardium in each ventricle, and the proportion of that volume consisting of muscle. The results showed a highly significant increase in overall ventricular size as a function of Carnegie stage and crown-rump length. Cardiac jelly was prominent in the early stages but was progressively replaced by the trabeculated muscle. The volume containing trabeculae had a consistent proportion of muscle, averaging 65%, for all stages after its appearance in stage 13. Curvature and thickness measurements of the compact part of the ventricles were made from images of the reconstructions. The mean curvature-thickness index (CTI) for the embryo hearts ranged from 0.24-0.61, and there was a significant increase in the index as a function of stage and crown-rump length.(ABSTRACT TRUNCATED AT 250 WORDS)