Quantitative determination of the development of collateral arteries and change in portal flow after an 80 per cent dearterialization of the liver of the rabbit was carried out with radioactive microsphere technique. The livers operated upon were classified macroscopically into two type groups. In the recovery type, the dearterialized lobes reverted to the preoperative state. In the necrotic type, necrosis in the dearterialized lobe progressed, and the nonarterialized lobe was compensatively hypertrophied. The rate of rearterialization into the dearterialized lobe was different according to the type. In the recovery type, collateral arteries developed rapidly until the fourteenth postoperative day, when the arterial flow reached 55 per cent and, thereafter, increased gradually. In the necrotic type, rearterialization progressed more rapidly, and arterial flow exceeded the preoperative value in the third postoperative week and, thereafter, decreased. In the necrotic type, portal flow in the dearterialized lobe had been significantly impeded within 12 hours after the operation. The state of portal flow in the dearterialized lobe is, we believe, a more important factor for the prognosis after the hepatic dearterialization than the rate of rearterialization or oxygen concentration of portal blood. High activity of serum glutamic oxalacetic transaminase and serum glutamic pyruvic transaminase of more than 90 units per milliliter on the seventh postoperative day can be an indicator for the impeded portal flow in the dearterialized lobe.