Metabolic zonation in the liver parenchyma, i.e., heterogene-hepatocytes, gradually declining toward the pericentral ones. 3,4 For the detoxification of ammonia, the liver contains both ureaity of the hepatocytes along the portocentral radius of the liver lobule, has been studied extensively. 1,2 The metabolism of and glutamine-synthesizing enzyme systems that have a reciprocal distribution pattern. In adult rat liver, carbamoyl phosamino acids for example, occurs predominantly in the periportal phate synthetase I (CPS), which is involved in the production of urea via the ornithine cycle, is present periportally, with a portal-to-central gradient over the liver lobule. This accounts other ornithine cycle enzymes in the rat are not known.