A new method is described for estimating the contributions of nitrate-N, newly-reduced nitrate-N and remobilization of stored nitrogen (N) for new growth of non-leguminous plants. Xylem sap is extracted from ¹⁵N-labelled and unlabelled plants. Nitrate-N and amino-N in sap are separated using cation-exchange resin and analysed for ¹⁵N. Remobilization of stored N is estimated using [I-(¹⁵N enrichment of amino-N/¹⁵N enrichment of nitrate-N)] × amino-N. Newly-reduced nitrate-N is estimated by difference. Field-application of this method is described.