The elimination of parenterally administered 6-azacytidine, 100 mg per kg of body wt, was studied in the urine of mice, rats, guinea pigs, rabbits, cats and dogs. The deamination of 6-azacytidine in vivo and in vitro is extremely low in rats, Within 24 hr the drug is completely eliminated in all investigated species, either in the original or the deaminated form. The deaminated product, 6-azauridine, accounts for about one-third of the total amount. In mice, the kidney is the organ with the highest enzymatic activity for the deamination of 6-azacytidine; in guinea pigs, the kidney and ileum; in rabbits, the ileum; in cats, the ileum and liver, while in dogs only the liver appears to be involved. The results are discussed from the point of view of 6-azauridine being the effective metabolite and from a phylogenetic aspect.