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A dynamic infusion method, originally developed for the pharmacokinetic studies of Iodoxamic acid®, was applied to the kinetic studies of the biliary excretion of another cholecystographic agent, iopanoic acid. This dynamic method has an important advantage in that the pharmacokinetic parameters involved in the hepatic uptake or biliary excretion can be evaluated from a single infusion experiment. Using the equilibrium dialysis technique, iopanoic acid was found to be highly bound to the plasma proteins. A linear relationship was found when the logarithm of unbound plasma concentration of iopanoic acid was plotted vs. the logarithm of its blood concentration. When the biliary excretion rates of iopanoic acid were fitted by a computer to the Michaelis-Menten equation against its unbound plasma concentration, the average Vm value was found to be 0.85 μM/kg/min and the average Km value was found to be 0.253 μM. Iopanoic acid was found to exist in monkey blood as unchanged species and in the bile mainly as the ester glucuronide.