Ticrynafen, a uricosuric diuretic agent which causes hepatocellular injury in man as an apparent idiosyncratic reaction, was found to impair the function of the isolated perfused rat liver. At concentrations in the perfusate equivalent to those produced in the blood of man by therapeutic doses, the drug led to a striking reduction in bile flow and sulfobromophthalein excretion and release of aspartate aminotransferase into the perfusate. These adverse effects were enhanced by treatment of rats with phenobarbital prior to removal of the liver, indicating that the adverse effect of ticrynafen is probably caused by a metabolite produced in the cytochrome P-450 system.