Studies have been made of the role of NADPH and NADH in carboxylation of alpha-ketoglutarate to isocytrate in the cytoplasm and mitochondria of the brain and liver in ontogenesis of hens. In both fractions of the liver, the reaction in the presence of NADPH is more intensive than that in the presence of NADH, being more intense in the cytoplasm than in mitochondria; during embryogenesis, the rate of the reaction increases up to hatching, decreasing at later stages of ontogenesis. No significant changes in the rate of carboxylation were observed during ontogenesis in the presence of NADH. In the brain, the reaction rate in the presence of NADH is higher than that in the liver, being lower in the presence of NADPH. In contrast to that in the liver, NADPH-dependent reaction in the brain is more intensive in mitochondrial fraction. The rate of both reactions, after the initial increase in embryogenesis, decreases at early postnatal stages, except NAD-dependent reaction which is sharply increased in brain cytoplasm of adult hens.