The enhanced dark CO(2) uptake after a preillumination period under varying O(2) concentrations has been measured with maize, a C(4) plant. For comparison the same study has been conducted with tomato, a C(3) plant. Increasing the O(2) concentration during preillumination inhibits by 70% the subsequent dark CO(2) uptake in tomato but stimulates 2-fold this CO(2) uptake in maize. The O(2) enhancement of CO(2) uptake in maize is due to the enhancement of malate and aspartate synthesis. The percentages of radioactivity incorporated in the C-4 of malate and aspartate vary from 74 to 87% when O(2) concentration during preillumination is increased from 0 to 100%.An interpretation of these data led to the conclusion that malate and aspartate formed during the light-enhanced dark fixation result from two competing carboxylation reactions: a single carboxylation of phosphoenolpyruvate and a double carboxylation involving ribulose 1,5-bisphosphate and phosphoenolpyruvate carboxylases. This latter route of carbon flow prevails when preillumination is carried out anaerobically. It appears that increasing O(2) concentration stimulates phosphoenolpyruvate synthesis during preillumination and consequently favors the single carboxylation of phosphoenolpyruvate, leading to an increased percentage of radioactivity in the C-4 atom of malate and aspartate.