When salicylazosulfapyridine (SAS) is fed to conventional rats, none of the drug is re covered in the urine, feces or cecum; the excreta contain tile products of azo bond re duction, sulfapyridine and 5-aminosalicylate and their metabolites in yields of greater than 50%. When tile intestinal microflora of conventional rats is decreased by the admin istration of neomycin, unchanged SAS is recovered in the cecum and feces. When an oral (lose of SAS is given to germ-free rats, recoveries of tile drug in the feces are over 50% whereas the urine contains an additional 1 to 2%. None of the metabolites of SAS char acteristic of conventional rats is found in the excreta of germ-free rats. When germ-free rats are infected with four specific bacteria normally found in the intestinal tract of ro dents, SAS is metabolized as in conventional rats. Bacterial strains representative of those found in the intestinal tract of the human and the rodent were cultured in the presence of SAS and in all cases azo bond reduction was detected. These findings indicate that tile intestinal bacteria are responsible for the initial reaction in SAS transformation and raise questions concerning the amount of intact SAS that reaches the presumed site of its action in inflammatory disease of the lower intestine.