In uricotelic organisms, the IMP catabolic pathway serves not only for purine nucleotide degradation and excretion, but also for excretion of excess nitrogen from ammo acids in the form of uric acid. This results in the differences in properties and amounts of enzymes committed in IMP degradation and hypoxanthine reutilization, as well as in the mode of regulation of these enzymes, between uricotelic and ureotelic organisms. For example, in birds, IMP-GMP 5` -nucleotidase activity is higher and hypoxanthine phosphoribosyltransferase activity is lower than those in mammalian liver. This seems to be critical in uricotelic organisms to shunt IMP toward the formation of uric acid. Regarding xanthine dehydrogenase, the Km for NAD of the chicken enzyme is lower than that of the rat enzyme. This appears to act as competition for NAD with the other NAD-dependent enzymes, under conditions of increasing importance in the purine catabolic pathway. In the liver of the chicken adapted to high dietary protein intake, there are concurrent increases in the activities of IMP-GMP 5` -nucleotidase, purine-nucleoside phosphorylase and xanthine dehydrogenase. This is not observed in rat liver.