Experimental evidence bearing on the hepatic uptake and excretion of sulfobromophthalein and bilirubin is considered. From this examination it is postulated that the hepatic handling of each compound consists of three steps: the uptake at the sinusoidal surface of the parenchymal cell by a concentrative membrane transport system of high capacity; the conjugation within the cell with glutathione and glucuronic acid, respectively; and the excretion into the biliary canaliculus of the respective conjugates by a second concentrative system of relatively low capacity. Two of the operating characteristics of this linked system, the easy attainment of saturation of the biliary excretory transport system and the presence of an intracellular storage phenomenon, have been shown to result from the disproportionate capacities of the two sequentially placed concentrative membrane transport systems. The disorders which may occur in the three steps underlying the handling of each of these compounds are also considered.