In hydroponically grown Lycopersicon esculentum (L.) Mill. cv. F144 the site of NO3− reduction and assimilation within the plant was shifted from the shoot to the root by salinity. Uptake of NO3− from the root solution was strongly inhibited by salinization. Consequently, NO3− concentrations in the leaf, stem and root tissues as well as the nitrate reductase activities of the leaves were lower in salinized than in control plants. Lower NO3−, but higher reduced-N, concentrations were observed in the xylem sap as a result of the enhanced participation of the root in NO3− reduction in salinized plants. Lower stem K+ concentrations and leaf malate concentrations were found in salinized compared to control plants which indicates reduced functioning of the K+−shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determined by supplying a pulse of NaH14CO3 followed by extraction and separation of the labelled products on ion exchange resins. The rate of H14CO3− incorporation was c. 2-fold higher in control than in salinized plants. In salinized plants the products of H14CO3− incorporation within the roots were diverted into amino acids, while the control plants diverted relatively more 14C into organic acids. Products of inorganic carbon incorporation in the roots of salinized plants provide an anaplerotic source of carbon for assimilation of reduced NO3− into amino acids, while in control plants the products were predominantly organic acids as part of mechanisms to maintain ionic balance in the cells and in the xylem sap.