Changes in the activities of glucose-6-phosphate dehydrogenase [EC 1.1.1.49], 6-phosphogluconate dehydrogenase [EC 1.1.1.44] and cytoplasmic and mitochondrial "malic" enzyme [EC 1.1.1.40] and NADP+- linked isocitrate dehydrogenase [EC 1.1.1.42] were measured in the liver, heart, lung and brain during ontogenesis in the chicken. In the liver the cytoplasmic malic enzyme was constant during embryonal development, increasing suddenly and markedly thereafter and isocitrate dehydrogenase increased in the embryo and decreased after hatching while their mitochondrial isoenzymes showed parallel but less marked changes. Activities of the other dehydrogenases were essentially unchanged. In the heart only cytoplasmic isocitrate dehydrogenase showed important changes, increasing three-fold during growth after hatching. In the lung, glucose-6-phosphate dehydrogenase and cytoplasmic malic enzyme attained their maximum activities respectively at 16 to 18 d and 14 d of development. Mitochondrial malic enzyme did not change, while isocitrate dehydrogenase reached its maximum between 14 and 18 d. In the brain cytoplasmic malic enzyme was activated only after hatching, while its mitochondrial isoenzyme and isocitrate dehydrogenase showed discontinuous variations of an insignificant magnitude. Other activities were unchanged.