The histoenzymic pattern of oxidative enzymes (G-6-PDH, G-PDH, ICDH, SDH, HBDH, NADH-2:tetrazolium dehydrogenase) was investigated in the developing neuroglia of rabbit brains, with special regard to the period of myelinogenesis. The obtained results lead to following conclusions: (1) During the early period of postnatal development there is maximal oxidative enzyme activity in ependymal cells, somewhat less reactive are the undifferentiated matrix cells and the differentiating cells of the mantle layer. No distinction can be made between the response of spongio- and neuroblasts. (2) Distinctly increased oxidoreductase activity, as compared to the early period of postnatal development, is demonstrated by the differentiating cells of myelination gliosis, no prevalence being demonstrable for enzymes of the particular metabolic pathways (pentose shunt, glycolysis or Krebs cycle). (3) G-6-PDH, G-PDH and oxidoreductases acting within the citric acid cycle are demonstrable only in single cells of the interfascicular oligodendroglia of adult rabbit brains, while almost all cells exhibit appreciable activity of HBDH and NADH-2 tetrazolium dehydrogenase.