Dark-grown barley leaves (Hordeum vulgare L.) were exposed to varying periods of illumination, followed by the supply of 14CO2 for 14 min in light or in darkness. Alcoholic extracts of the leaves were analyzed by chromatography. It was found that leaves without previous illumination were able to synthesize only malate, aspartate, glutamate, citrate, and succinate, equally in light or darkness. The products of dark fixation of 14CO2 were not affected by the length of prior illumination. In light, after 2 to 4 h of illumination, the synthesis of malate, aspartate, and glutamate was greatly accelerated. At this time intermediates of the Calvin cycle began to appear, followed by a rapid increase in sucrose synthesis. After more than 6 h of illumination, sucrose became the main photosynthetic product. A kinetic examination of the carbon flow into intermediates of carboxylation in light and in darkness was also conducted.The results indicate that during chloroplast development upon illumination there is a successive activation of distinct components of photosynthesis. The first photosynthetic reaction is a light-dependent β-carboxylation. This is followed by the development of the Calvin cycle, and the glycolate pathway. Dark CO2 fixation appears to be a different process from light-requiring β-carboxylation, located in a separate part of the cell.