CO2 exchange, (14)CO2 fixation and radioactive products of Chlorella vulgaris (strain 211-11f) were examined during the induction period at temperatures of +10° and +35°C. The algae were grown under low CO2 concentration (0.03 vol.-%) at a temperature of +27°C. The formation of transients in CO2 uptake in Chlorella, measured with an infrared gas analyser, is highly dependent on the temperature used during the measurements. At higher temperatures (+15°C and above) a maximum in CO2 uptake exists at the beginning of the illumination period followed by a minimum before a steady rate of photosynthesis is reached. At a temperature of +10°C, on the other hand, a long lag phase without a minimum in CO2 uptake could be observed which also occurs in the time course of (14)CO2 fixation.The autoradiographic studies of the kinetics of the appearance of labelled products at a temperature of +10°C showed that at the beginning of the light period nearly all of the radioactivity was incorporated into malate and aspartate. Under these conditions intermediates of the Calvin cycle were labelled after an illumination time of 2 minutes. At a temperature of +35°C radioactivity appeared in 3-phosphoglycerate as well as in malate and aspartate after a photosynthetic period of 10 sec. At this temperature 3-phosphoglycerate not only appeared earlier than at +10°C but it was also more strongly labelled. Moreover the intermediates of the Calvin cycle were labelled after 20 or 30 sec of photosynthesis. These results show that a carboxylation of phosphoenolpyruvate and the Calvin cycle are involved in the formation of the transients of CO2 uptake in Chlorella vulgaris. The possible role of the control of photosynthetic carbon metabolism during the induction period is discussed.