The isolated perfused liver, free from endogenous regulators of bile flow, excretes bile at a uniform rate. This allows investigation of the composition of the bile fraction stimulated by secretin. Thirteen pig livers were perfused with or without sodium taurocholate. After collection of control bile, secretin was introduced by constant infusion or by single injection. Calculation of the anion concentrations of the secretin-stimulated fraction of bile revealed that these concentrations remained within narrow limits regardless of secretin dose. Failure of total bile flow to influence these concentrations suggested that ion exchange or reabsorptive mechanisms in the biliary tree were not operative in the secretin-stimulated fraction of bile.