Hepatocytes differ in their metabolism depending on their position in the liver acinus. To assess how such specialization changes during development, we used quantitative cytochemistry to measure succinate dehydrogenase (SDH), glutamate dehydrogenase (GDH), glucose-6-phosphatase (G6P), and NADPH dehydrogenase (ND) activities specifically in periportal and perivenular hepatocytes in developing rats, aged between 1-114 days. Important and distinct changes were observed in each zone for each enzyme during development. An intra-acinar gradient of distribution was present from day 1 for SDH and G6P and from day 5 for GDH and ND. It was being similar to the adult value for SDH but less pronounced for the remaining enzymes. The SDH and G6P activity was greater in periportal cells, and the GDH and ND activity was greater in perivenular cells. The more pronounced distribution with age was due, for G6P, to an initial specific periportal increase combined with a mild perivenular decrease and for GDH to a greater perivenular than periportal increase. The ND first increased simultaneously in both zones, but from day 20 the perivenular increase became prevalent. The SDH changes were parallel in both zones. All zonal enzyme activities changed distinctly after weaning. To what extent the changes in activities and metabolic zonation observed in our study reflect a response to specific metabolic demands of the liver or can be modified by environmental factors remains to be investigated.