The distribution of activity of NAD . P-dependent isocitrate dehydrogenase in the chick liver and brain cytoplasm and mitochondria was studied during ontogenesis. A marked increase of its activity was found in both liver fractions during embryogenesis, with the predominance in mitochondria; the major part of its activity during postembryonic development is localized in the cytoplasm. In the brain tissues where this enzyme is less active in both the factions its activity increased to a greater extent in the mitochondria and exceeded markedly that in the cytoplasm.