We used Doppler echocardiography to quantitate the changes in intracardiac blood flow velocities and right and left ventricular stroke volumes in 80 normal human fetuses from 19 to 40 weeks gestation. Blood flow velocity spectra across the aortic, pulmonary, tricuspid, and mitral valves were digitized to obtain peak velocities (m/sec) and flow velocity integrals. Aortic and pulmonary diameters were measured at valve level from two-dimensional echocardiographic images and cross-sectional area was calculated assuming a circular orifice. Ventricular stroke volume was calculated as the product of the cross-sectional area of a great vessel and the flow velocity integral through that vessel. The pulmonary arterial and aortic diameters increased linearly with gestational age (r= .82,r= .84), and pulmonary arterial diameter consistently exceeded aortic diameter. There was a positive relationship between stroke volume and gestational age: stroke volume increased exponentially from 0.7 ml at 20 weeks to 7.6 ml at 40 weeks for the right ventricle (r= .87) and from 0.7 ml at 20 weeks to 5.2 ml at 40 weeks for the left ventricle (r= .91). Similar results were obtained for right and left ventricular and combined cardiac outputs. In 44% of the fetuses it was possible to quantitate both right and left ventricular stroke volumes. There was a close correlation between right and left ventricular stroke volumes in these fetuses (r= .96) and right ventricular stroke volume exceeded left ventricular stroke volume by 28%. Flow velocity across the tricuspid and mitral valves was consistently greater during atrial systole (A wave) than during rapid ventricular filling (E wave) (0.52 0.07 vs 0.37 + 0.08 m/sec and 0.45 ± 0.07 vs 0.33 ± 0.06 mlsec). The E/A ratios for the mitral and tricuspid valves were similar throughout the period of gestation studied, indicating equivalent diastolic ventricular function. This study demonstrates that right and left ventricular stroke volumes increase by approximately 10- fold from 20 to 40 weeks gestation in the normal human fetus. It also demonstrates, within the limitations of the equipment, that right ventricular stroke volume exceeds that of the left ventricle, thus confirming right ventricular dominance in utero.