The plasma clearance and extraction ratio of indocyanine green (ICG) were followed in cats anesthesized with chloralose. In one series of experiments the ICG infusion rate was raised stepwise and in another the hepatic plasma flow was changed by replacing part of the blood volume with erythrocytes, plasma, or dextran (Macrodex). For unknown reasons blood replacement with Macrodex increased the ICG elimination. Both plasma clearance and extraction ratio were independent of ICG infusion rate but dependent on hepatic plasma flow. In contrast, a clearance calculated from the dye elimination rate and the estimated average hepatic plasma concentration of ICG was independent of changes in hepatic plasma flow. In conclusion, plasma clearance and extraction ratio of indocyanine green are not ideal for estimation of liver function. However, when clearance is calculated from the average hepatic plasma concentration of the dye, it becomes independent of hepatic plasma flow and thus a more reliable index of liver function.