A mass spectrometer with a membrane inlet system was used to monitor directly gaseous components in a suspension of algae. Using labeled oxygen, we observed that during the first 20 seconds of illumination after a dark period, when no net O(2) evolution or CO(2) uptake was observed, O(2) evolution was normal but completely compensated by O(2) uptake. Similarly, when CO(2) uptake was totally or partially inhibited, O(2) evolution proceeded at a high (near maximal) rate. Under all conditions, O(2) uptake balanced that fraction of the O(2) evolution which could not be accounted for by CO(2) uptake.From these observations we concluded that O(2) and CO(2) are in direct competition for photosynthetically generated reducing power, with O(2) being the main electron acceptor during the induction process and under other conditions in which CO(2) reduction cannot keep pace with O(2) evolution. The high rate of the O(2) uptake reaction observed in the presence of iodoacetamide, KCN, or carbonyl cyanide p-trifluoromethyoxyphenylhydrazone, suggests that a special high capacity oxidase distinct from ribulose diphosphate oxygenase exists in whole cells. The rapid reduction of molecular O(2) after a period of darkness probably serves as a priming reaction for the photosynthetic apparatus. The high steady state rate of the O(2) cycle in the absence of CO(2) fixation suggests that the regulation of photosynthesis does not involve significant changes in the rate of photochemical electron transport.