The biotransformation of the 131I-labeled cholegraphic media ioglycamic acid, iodoxamic acid and iotroxic acid in man is investigated. Plasma, urine and fistular bile were analyzed for unchanged and metabolized constituents of the administered substances using thin layer chromatography. No metabolites were found in plasma, but up to two were found in urine in addition to unchanged contrast media (a total of 50% of the total elimination in 24 hr. urine). A metabolite was only found in the fistular bile after the injection of iotroxic acid.