1. The uptake of bilirubin was studied in the perfused rat liver by a multiple-indicator dilution technique employing the three-compartment model of Goresky. 2. The kinetics of hepatic bilirubin uptake could be described by the Michaelis-Menten equation. 3. The maximal uptake velocity (V max.) and the apparent half-saturation constant (Km) were 4-4 +/- 0-5 nmol s-1 g-1 of liver and 58 +/- 16 nmol/g of liver respectively, indicating that the hepatic uptake system for bilirubin under normal conditions is operating far below saturation. 4. Sodium taurocholate did not compete with bilirubin for hepatic uptake. 5. These findings are consistent with the concept that carrier-mediated transport is responsible for hepatocellular uptake of bilirubin and that bilirubin and bile acids enter the hepatocyte via separate pathways.