Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO 3-content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wildtype controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO 3-content was observed in plants grown in nitrogenreplete conditions at low irradiance (170~tmol photons.m-2 .s-l). This relationship disappeared at higher irradiance (450~tmol photons.m-2.s-1) because the maximal extractable NR activity in the leaves of the wildtype plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO 3-accumulated in the leaves of plants grown at 450~tmol photons.m-2.s-~ than in those grown at 170 ~tmol photons.m-e.s-~ in N-replete conditions. The foliar NO 3-level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO 3-was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO 3-in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.