Against the background of high rates of nitrogen (N) input into forest ecosystems and, in part, high nitrate (NO 3 - ) concentrations in the soil solutions, NO 3 - reduction activity and N accumulation in leaves and roots of young pedunculate oaks (Quercus robur) were investigated. Seedlings with unrestricted root growth, and 2-year-old saplings with cut root-stocks, were grown hydroponically at different forms and concentrations of N. The nitrate reductase activity (NRA) of the leaves and fine roots was measured in vivo without addition (NRA H 2 O ) or with addition of exogenous NO 3 - (NRA H 2 O ) to the incubation assay. The amounts of reduced NO 3 - as calculated with the NRA H 2 O and NRA K N O 3 were compared with the uptake of 1 5 NO 3 - , and with root-to-shoot translocation of NO 3 - as determined by NO 3 - concentrations of the xylem sap and transpiration rates. Compared with the NRA H 2 O , the NRA K N O 3 was higher by a factor of approximately 10 in the current year's fine roots, and by a factor of about 60 in the leaves. In only one case did increased NO 3 - concentrations of the nutrient solution result in an increase in NRA. In some cases, NRA was diminished in the presence of ammonium (NH 4 + ) in the root medium. The quantities of reduced NO 3 - as calculated on the basis of NRA H 2 O agreed with the amounts of 1 5 N accumulated in roots and leaves, and with the amounts of NO 3 - translocated from the roots to the shoots. The contribution of the leaves to the total plant's NO 3 - reduction as computed on the basis of NRA H 2 O was 1-17% in the seedlings, but up to 86% in the saplings; here, it correlated significantly with the leaf/root ratios on a fresh-weight basis. Compared with the leaf/root ratios, the form and concentration of the supplied N had a much lower impact on the share of the leaves in NO 3 - reduction; and did not affect the foliar N concentrations. In the leaves as well as in the roots, the concentrations of soluble NO 3 - were very low (<0.5 mg NO 3 - N g dry weight - 1 ). The results show that young pedunculate oaks have a low affinity for NO 3 - N, even in the case of high NO 3 - supply.