Changes in liver blood flow immediately after and during one week after a standardized liver hilus dearterialization were studied in a series of 16 dogs. An additional six dogs were sham-operated and used as controls. The liver blood flow was measured using Xe133 injections administered by two methods in every animal: into the liver parenchyma, and into the portal vein. The measurements were taken immediately before, and 15, 30, 60 minutes, and one and seven days after the operation. Xe133 wash out from the liver was found to be reduced 15 minutes after the dearterialization, measured both by liver parenchymal and portal venous injection routes (p less than 0.05). Return to normal took place rapidly thereafter. On the next day some increase in the liver blood flow was observed in both dearterialized and sham-operated dogs. The aleration was significant in the dearterialized animals as measured by portal vein injections, and in the sham-operated dogs as measured by liver parenchymal injections (p less than 0.05). On the seventh day normal Xe133 liver wash out was recorded in all animals. It can be concluded that after liver hilus dearterialization in the dog liver blood flow was decreased for a very short period of time only. Increased flow was noted as early as the next day, not only in the dearterialized, but also in the sham-operated animals. The use of adequate controls is important.