Changes in the distribution of portal venous blood flow to the left and right lobes of the liver and through the ductus venosus were determined from before birth through 9 days of age in 25 chronically catheterized fetal and newborn lambs. Blood flow distribution was calculated by means of the radionuclide-labeled microsphere technique. With umbilical cord clamping portal venous blood flow distribution to the right lobe of the liver decreased when compared to that in the term fetus; distribution to the left lobe and ductus venosus increased. More than 50% of portal blood flow was shunted through the ductus venosus during the first hour after birth. Ductus venosus shunts of at least 25% of total portal blood flow were noted in half the lambs between the second and sixth days of life. Portal venous distribution to the right lobe was inversely related to the fraction of portal blood flow shunted through the ductus venosus. The ratio of right lobe weight to total liver weight was significantly correlated with the ratio of right lobe flow to total liver flow (r = 0.73; P less than 0.001). The conclusion is that large and persistent ductus venosus shunts are normal during the first postnatal week in lambs.