Limiting the supply of nitrate to winter-lettuce in greenhouses is one approach to assuring an acceptably low nitrate level in the marketed produce. To optimize growth under such conditions, a two-state-variable lettuce model, originally developed to predict the nitrate concentration when the supply of nutrients is unlimited (Seginer et al., 1998), has been modified to take account of growth limited by nitrate supply. Nitrate in the plant serves two functions: it is a vital raw material for protein synthesis and also an important osmoticum in the cell sap. The plant model responds to limited nitrate supply by placing a higher priority on its role as raw material. Nitrate in the cell sap is replaced, as much as needed, by assimilates (sugar), freeing the recovered nitrate for protein synthesis. The modified model includes a balance equation for nitrate in the plant dSNv/dt = FNrv - rFCvs , where SNv is nitrate-N content, FNrv is nitrate-N uptake by the plant, FCvs is the rate of structural growth and r is the nitrogen content of the structural material. dSNv/dt is determined by the rate of change of the assimilate content of the cell sap, as described by the original model. If nitrate supply is abundant, its rate of uptake, namely FNrv, is determined as a remainder. If nitrate supply is limiting, and therefore is all taken up by the plant, it is the nitrate used for growth, rFCvs, that is determined by the balance. Comparison with limited experimental results produced a good agreement.