The effect of 1% and 21% O(2) upon (14)CO(2) assimilation by desert plants exposed for 10 to 90 seconds has been studied. The plants studied can be divided into three groups with respect to O(2). The C(3) plants display the usual Warburg effect. No changes could be observed in the intensity of photosynthesis as a function of O(2) content in another group of plants (showing signs of Crassulacean acid metabolism). In still another group of plants (C(4) plants) the stimulating effect of O(2) on photosynthesis could be detected. In C(3) plants, O(2) inhibits the processing of carbon through the Calvin cycle intermediates. The involvement of carbon in the glycolate pathway fails to explain completely the inhibiting effect of O(2) on photosynthesis. It is assumed that O(2) inhibits the enzymes of the Calvin cycle. In C(4) plants O(2) stimulates the incorporation of (14)C into malate and aspartate. The incorporation of (14)C into the intermediates of the Calvin cycle in C(4) plants is inhibited much like that in typical C(3) plants.