The changes in the concentrations of FDP *, DAP, PGA, RuDP, PEP, Pyruvat, ADP and ATP in Chlorella occuring during the transition from light to dark and from dark zu light were enzymaticaly determined. Moreover after a period of photosynthesis, the rate of 14CO2 dark-fixation and the distribution of 14C in the intermediates were determined. During the first few sec after illumination in air+CO2 FDP, DAP and ATP increase. while PGA, PEP and ADP decrease. The opposite changes are observed during the light-dark-change. The pronounced increase in PGA is practically linear during the first 6 sec after the illumination is discontinued. When comparing to the CO2 fixation rate of the preceding period of photosynthesis no stoichiometrical relationship was obtained. Depending on the rate of photosynthesis CO2/PGA ratios between 0.3 and 3.5 were obtained. Absence of CO2 has no measurable influence on the rate of PGS formation after turning off the light, where as under N2 the formation of PGA is completely inhibited. It is concluded that the big changes in the concentration of the intermediates are caused by a competition of light phosphorylation with glycolysis for the inorganic phosphate anologous to the Pasteur effect in yeast. Beginning of illumination means beginning of light phosphorylation and decrease in the concentration of ΡΟ43⊖ and thus inhibition of triose oxidation. As a consequence PGA and PEP drop, while DAP and FDP accumulate. When illumination stops, light phosphorylation stops and thus ΡΟ43⊖ is increased and triose oxidation is accelerated. The result is a fast increase in PGA and a decrease in FDP and DAP. A small part of the PGA formed after turning off the light originates from the reaction of RuDP with CO2. It is assumed, that the carboxylation product of RuDP participating in photosynthesis is not identical with free PGA with which it therefore is not at equilibrium. By hydrolysis of the carboxylation product during extraction or after darkening, however, free PGA is formed.