A comparison has been made between the flow calculated from the initial slope of the 133Xe elimination curve registered over the liver after intraportal injection of the tracer and the simultaneously recorded total outflow from the liver. This was made possible by diversion of the blood from the hepatic veins to a flow recorder outside the animal. The results indicate that in the same animal there is a fairly constant relationship between the flow values even under different rates of flow. Between different cats this relationship varies. It is concluded that the flow values calculated from the initial slope of the 133Xe elimination curve is valid as far as relative changes in total liver blood flow is concerned.