The transport of sulfobromophthalein glutathione was studied in perfused livers isolated from phenobarbital treated and control rats. Phenobarbital increased the cell size and the uptake of sulfobromophthalein glutathione. The effect on uptake is specific since in phenobarbital treated livers each unit of hepatocyte surface area takes up more sulfobromophthalein glutathione than controls. The cellular hypertrophy does not involve all cell functions; total and specific content of cytosolic fatty acid binding protein for example, were unchanged by phenobarbital. The increase in Vmax and influx rate constant for sulfobromophthalein glutathione uptake suggest that phenobarbital increases the amount of membrane carriers or their rate of cycling.