Increases in atmospheric CO2 concentration ([CO2]) can lead to global climate change and theoretically could enhance carbon (C) deposition in soil, but data on this complex issue are contradictory. One approach for clarifying the diverse forces influencing plant-derived C in the rhizosphere involves defining how elevated [CO2] alters the fundamental process of C transfer from plant roots to the soil. We examine here how a step increase in [CO2] affects the innate influx and efflux components of root exudation in axenic plants, as one foundation for understanding how climate change may affect rhizodeposition. Increasing [CO2] from 425 to 850 μmol mol−1 during short-term trials enhanced shoot and root dry weight (P<0.01) of annual rye grass (Lolium multiflorum Lam.) and medic (Medicago truncatula L.) but had no effect on growth of maize (Zea mays L.). Root amino-acid flux in the same plants changed only in maize, which increased the efflux rate (nmol g root fresh weight−1 h−1) of six amino acids (arginine, alanine, proline, tyrosine, lysine and leucine) significantly (P<0.05) under elevated [CO2]. None of the three plant species altered the steady-state concentration of 16 amino acids released into a hydroponic solution with changing [CO2], apparently because amino-acid influx rates, measured at 2.5 μm, consistently exceeded efflux rates. Indeed, plants recovered amino acids at rates 94–374% higher than they were lost from roots regardless of [CO2]. These results indicate that, in theory, any effect of [CO2] doubling on amino-acid efflux can be offset by innately higher rates of influx. In practice, however, higher rates of amino-acid cycling (i.e., efflux+influx) for each root segment (in C4 maize) or from more root tissue (in the two C3 species) should increase root exudation by plants exposed to elevated [CO2] as additional amino acids would be adsorbed to soil particles or be taken up by soil microorganisms.