1. The secretion of thyroxine glucuronide into bile has been investigated in the isolated perfused rat liver and also in the intact animal. Radioactive material for the study was prepared from the bile of rats treated with [(125)I]thyroxine.2. The isolated liver actively transports thyroxine glucuronide from the perfusion fluid (Tyrode solution) into the bile. At equilibrium, its concentration in the bile is about 1800 x its concentration in the perfusion fluid.3. Experiments with pulse infusions of glucuronide indicate that the substance enters the bile almost as soon as it is captured by the tissue.4. Secretion of thyroxine glucuronide is strongly inhibited by bromosulphophthalein, indicating a common carrier mechanism. It is suggested that secretion probably involves two stages - transfer from the perfusion fluid into the cell and then from the cell into the bile. The second stage is certainly active.5. When thyroxine glucuronide is injected intravenously into anaesthetized rats, some 77% is recovered from the bile, half of this being excreted within the first 14 min.