Wheat was cultivated in a small phytotronic chamber. (18)O(2) was used to measure the O(2) uptake by the plant, which was recorded simultaneously with the O(2) evolution, net CO(2) uptake, and transpiration. At normal atmospheric CO(2) concentration, photorespiration, measured as O(2) uptake, was as important as the net photosynthesis. The level of true O(2) evolution was independent of CO(2) concentration and stayed nearly equal to the sum of net CO(2) photosynthesis and O(2) uptake. We conclude that at a given light intensity, O(2) and CO(2) compete for the reducing power produced at constant rate by the light reactions of photosynthesis.