The influence of variation in the concentration of dissolved inorganic carbon (DIC) in the form of CO2 and HCO3− in the root media on the C and N metabolism of Lycopersicon esculentum cv. F144 was investigated under both saline and non-saline conditions. Tomato seedlings were grown in hydroponic culture (pH 6.5) with or without NaCl, and the root solution was aerated with either ambient CO2 (360 μmol mol−1) or CO2-enriched air (5000 μmol mol−1). Nitrate uptake and root tissue NO3− concentrations were increased slightly by elevated rhizosphere DIC concentrations in both control and salinity-treated plants. This was associated with 46% higher nitrate reductase activity in the roots of control plants supplied with elevated DIC than in those supplied with ambient DIC. The activity of phosphoenolpyruvate carboxylase (PEPc) in vitro in control and salinity-treated plants was unaffected by the supply of elevated rhizosphere DIC concentrations. However, PEPc activity in vitro was considerably higher than the rates of PEPc activity in vivo reported previously, indicating that PEPc activity was not in itself a limitation on the provision of anaplerotic C. Therefore elevated DIC concentration in the rhizosphere stimulated the uptake of NO3− and provided alternative C skeletons for the assimilation of the NH4+ resulting from NO3− reduction into amino acids within the roots. Salinity stimulated root glutamine synthetase (GS) activity up to double that in control plants. Furthermore, elevated DIC caused an increase in leaf and root GS activity of control plants while inhibiting GS activity in the roots of salinity-treated plants. Glutamine[ratio ]2- oxoglutarate aminotransferase (GOGAT) activity of salinity-treated plants was doubled by elevated rhizosphere DIC concentrations. These changes in GS and GOGAT activity must reflect changes in amino acid synthesis. Under saline conditions the xylem transport of NO3− is partly blocked and a larger root assimilation develops, requiring not only the transamination of 2-oxoglutarate to glutamate but also that of oxaloacetate to aspartate and the transamidation of aspartate to asparagine.