Six genotypes of winter wheat (Triticum aestivum L.) differing in grain protein concentration were grown on a nutrient solution containing low concentrations of NO(3) (-) (2 millimolar). Total NO(3) (-) uptake varied between genotypes but was not related to grain protein content. An in vivo nitrate reductase assay was used to determine the affinity of the enzyme for NO(3) (-), and large phenotypic variations were observed. In vivo estimations of the concentration and size of the metabolic pool were variable. However, the three genotypes with the higher ratios of metabolic pool size to leaf total NO(3) (-) concentration were the high protein varieties. It is proposed that a high affinity of nitrate reductase for nitrate might be a biochemical marker for the capacity of the plant to continue assimilating NO(3) (-) for a longer period during the last stage of growth.The potential use of such physiological criteria as markers is discussed, in particular with respect to breeding programs for the development of plants with efficient nitrogen utilization.