The end product of purine metabolism varies from species to species. The degradation of purines to urate is common to all animal species, but the degradation of urate is much less complete in higher animals. The comparison of subcellular distribution, intraperoxisomal localization forms, molecular structures, and some other properties of urate-degrading enzymes (urate oxidase, allantoinase, and allantoicase) among animals is described. Liver urate oxidase (uricase) is located in the peroxisomes in all animals with urate oxidase. On the basis of the comparison of intraperoxisomal localization forms, mol wt, and solubility of liver urate oxidase among animals, it is suggested that amphibian urate oxidase is a transition form in the evolution of aquatic animals to land animals. Allantoinase and allantoicase are different proteins in fish liver, but the two enzymes form a complex in amphibian liver. The subcellular localization of allantoinase and allantoicase varies among fishes. Hepatic allantoinase is located both in the peroxisomes and in the cytosol in saltwater fishes, and only in the cytosol in freshwater fishes. Hepatic allantoicase is located on the outer surface of the peroxisomal membrane in the mackerel group and in the peroxisomal matrix in the sardine group. Amphibian hepatic allantoinase-allantoicase complex is probably located in the mitochondria. On the basis of previous data, changes of allantoinase and allantoicase in molecular structure and intracellular localization during animal evolution may be as follows: Fish liver allantoinase is a single peptide with a mol wt of 54,000, and is located both in the peroxisomes and in the cytosol, or only in the cytosol. Fish liver allantoicase consists of two identical subunits with a mol wt of 48,000, and is located in the peroxisomal matrix or on the outer surface of the peroxisomal membrane. The evolution of fishes to amphibia resulted in the dissociation of allantoicase into subunits, and in the association of allantoinase with the subunit of allantoicase. This amphibian enzyme was lost by further evolution.