The rates of biliary and urinary excretion of iocetamic acid (Cholebrine®), iopronic acid (Oravue®), and iosumetic acid were compared in dogs with chronic bile-fistulas during stepwise increasing constant infusions of each contrast agent. A constant infusion of taurocholate was administered in each study at a rate of 0, 0.5, 2.0 or 3.0 μmol/min/kg. The calculated maximum rates of biliary excretion (Emax) of iocetamic, iopronic, and iosumetic acid were not significantly different. The choleretic effect of iopronic acid (17.2 μl/μmol) was approximately twice that of iocetamic acid (9.7 μl/μmol or iosumetic acid (8.6 μl/μ mol). As a result, the maximum concentrations of iocetamic acid (59.6 μmol/ml) and iosumetic acid (61.8 μmol/ml) in bile during infusion of taurocholate at 0.5 μmol/min/kg were significantly higher than that of iopronic acid (36.3 μmol/ml). Only a fraction of the injected contrast media were cleared by the kidneys (0.34, 0.43, and 0.14 ml/min/kg for iocetamic, iopronic, and iosumetic acid, respectively). Biotransformation of each contrast medium found in bile was extensive, with only a small fraction of total contrast medium remaining as unchanged parent compound. The uronic acid content suggested that, in bile, the glucuronide was the major metabolite in all three media. Taurocholate infusion enhanced the Emax of the three contrast agents by 0.32 μmol per μmol of bile salt. At low rates of bile salt excretion, the Emax. of iocetamic, iopronic and iosumetic acid were similar or slightly higher than the previously reported Emax of iopanoic acid. At bile salt excretion rates of 2 μmol/min/kg and higher, the Emax of the three contrast media were less than that of iopanoic acid. Thus, in terms of hepatic excretion iopanoic acid seems to be a better cholecystographic agent than iocetamic, iopronic or iosumetic acid in patients with circulating bile salts (fed patients).