The course of the venous circulation in the fetal liver has been studied in fetal lambs by means of the radionuclide-labeled microsphere technique. About 50% of umbilical venous blood passes through the ductus venosus, while the remainder is distributed to both lobes of the liver. Portal venous blood is largely distributed to the right lobe of the liver, with a small proportion passing through the ductus venosus and none to the left lobe. Because of these flow patterns, oxygen saturation is lower in the right than in the left hepatic vein. Left hepatic venous blood joins the ductus venosus stream and these preferentially pass through the foramen ovale, whereas right hepatic venous blood joins the distal inferior vena caval stream and preferentially passes through the tricuspid valve. These patterns favor distribution of well-oxygenated blood to the fetal heart and brain. Hypoxia and reduced umbilical venous return are associated with reduced flow through the hepatic microcirculation with proportionately greater ductus venosus flow. In the fetus, the liver has a major role in influencing venous return to the heart and in regulating distribution of oxygen and energy substrate supply to different fetal organs.