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Two choleretic agents, taurocholate and cinchophen, were studied in the dog to determine their effect on both the biliary and urinary excretion of two related cholecystographic agents, iopanoic add and iophenoxic acid. Liver concentrations were determined on biopsies. Trace amounts of radioactively labeled compounds were used for measurements. The effect of the choleretic agent was specific for the cholecystographic compound. Taurocholate increased both biliary and urinary excretion of iopanoic acid and decreased its intrahepatic level, but had no effect on iophenoxic acid. Cinchophen had no effect on iopanoic acid but increased both biliary and urinary excretion of iophenoxic acid. When examined against a pre-existing high rate of bile flow, the active choleretic increased, rather than decreased, the biliary concentration of the cholecystographic compound. The increased excretion in both bile and urine cannot be attributed simply to the lessened concentration gradient from plasma to bile, as proposed by others. The data are consistent with a direct action of the choleretic agent on hepatic mechanisms, either to decrease intracellular binding or to alter the characteristics of transport.