We present a method for chronic catheterization of the hepatic veins of fetal lambs in utero. A detailed description of hepatic venous anatomy, essential to the technique, is also presented. Right, left, or both hepatic veins were catheterized successfully in 23 of 28 animals. We assessed catheter position accurately during surgery by comparing oxygen saturations in the hepatic vein, descending aorta, umbilical vein, and inferior vena cava. Right hepatic venous saturation usually closely resembles that in the descending aorta, whereas left hepatic venous saturation lies between aortic and umbilical venous saturations. Samples obtained through these catheters are representative of hepatic venous blood and are not contaminated by reflux from the inferior vena cava. These small catheters are unlikely to alter the distribution of blood within the liver or streamlining of blood within the thoracic inferior vena cava. When this technique is coupled with catheterization of the descending aorta, umbilical vein, and portal vein, the preparation allows quantitation of blood flow, oxygen consumption, and metabolism in the liver of the unanaesthetized fetal sheep in utero.