Sulfobromophthalein (BSP) was used to test the applicability of the isolated perfused rat liver in the study of drug metabolism. Flow of perfusate through the liver averaged 35 ml per min and the bile flow rate was 0.75 microliters per min per g of liver. BSP was delivered into the system as a single dose of 4.7 to 9.5 mumoles. Samples of perfusate from the pre and posthepatic circuits and bile samples were obtained at different time intervals. The following parameters were elimination half life, mean hepatic clearance, mean intrinsic clearance, time course of hepatic and bile concentrations and percent dose excreted into bile. The results demonstrated that bile flow and BSP biliary excretion depended on hepatic blood flow. Time course of hepatic concentration decay and mean intrinsic clearance were a reflection of the inherent ability of the liver for BSP elimination into bile. The parameters tested and their correlations gave overall information on isolated liver function that may be applied to the study of other exogenous or endogenous compounds.