Effects of Na application on the capacity of NO(3) (-) assimilation were studied in Na-deficient Amaranthus tricolor L. cv Tricolor plants. On day 30 after germination, Na-deficient A. tricolor plants received either 0.5 millimolar NaCl or KCl. The level of nitrate reductase activity doubled within 24 hours by the addition of Na and the enhanced level was maintained thereafter. When the plants were exposed to 2 millimolar (15)NO(3) (-), total (15)N taken up by the plants was greater in the Na-treated plants than in the K-treated plants within 24 hours of the Na treatment. Incorporation of (15)N into the 80% ethanol-insoluble nitrogen fraction of the Na-treated plants in the light period was about 260% of those of the K-treated plants indicating greater capacity of NO(3) (-) assimilation in the Na-treated plants. From these results, it was demonstrated that Na application to the Na-deficient A. tricolor plants promoted NO(3) (-) reduction and its subsequent assimilation into protein, resulting in growth enhancement.