Plants grown at elevated CO2 often acclimate such that their photosynthetic capacities are reduced relative to ambient CO2-grown plants. Reductions in synthesis of photosynthetic enzymes could result either from reduced photosynthetic gene expression or from reduced availability of nitrogen-containing substrates for enzyme synthesis. Increased carbohydrate concentrations resulting from increased photosynthetic carbon fixation at elevated CO2 concentrations have been suggested to reduce the expression of photosynthetic genes. However, recent studies have also suggested that nitrogen uptake may be depressed by elevated CO2, or at least that it is not increased enough to keep pace with increased carbohydrate production. This response could induce a nitrogen limitation in elevated-CO2 plants that might account for the reduction in photosynthetic enzyme synthesis. If CO2 acclimation were a response to limited nitrogen uptake, the effects of elevated CO2 and limiting nitrogen supply on photosynthesis and nitrogen allocation should be similar. To test this hypothesis we grew non-nodulating soybeans at two levels each of nitrogen and CO2 concentration and measured leaf nitrogen contents, photosynthetic capacities and Rubisco contents. Both low nitrogen and elevated CO2 reduced nitrogen as a percentage of total leaf dry mass but only low nitrogen supply produced significant decreases in nitrogen as a percentage of leaf structural dry mass. The primary effect of elevated CO2 was to increase non-structural carbohydrate storage rather than to decrease nitrogen content. Both low nitrogen supply and elevated CO2 also decreased carboxylation capacity (Vcmax) and Rubisco content per unit leaf area. However, when Vcmax and Rubisco content were expressed per unit nitrogen, low nitrogen supply generally caused them to increase whereas elevated CO2 generally caused them to decrease. Finally, elevated CO2 significantly increased the ratio of RuBP regeneration capacity to Vcmax whereas neither nitrogen supply nor plant age had a significant effect on this parameter. We conclude that reductions in photosynthetic enzyme synthesis in elevated CO2 appear not to result from limited nitrogen supply but instead may result from feedback inhibition by increased carbohydrate contents.