Cuttings of a poplar clone (Populus x euramericana 'Dorskamp') were exposed to filtered air and to filtered air with 80 to 135 nl 1(-1) NO2 added in climate chambers during 12 weeks. Three different levels of nitrogen fertilization were used, the lowest causing symptoms of nitrogen deficiency. NO2 fumigation caused no visible injury to the plants. Fumigated plants showed elevated activity of nitrate reductase and higher leaf nitrogen concentrations relative to the control, indicating nitrogen assimilation from NO2. Fumigation enlarged the foliar area and, at the lowest nitrogen supply from the substrate, elevated the net CO2 assimilation rate. At the highest level of soil nitrogen supply, fumigation enlarged the width of xylem and bark tissue in the main stem. Fumigation had a stimulating effect on total biomass production during the exposure period. Thus, NO2 acted as a nitrogen fertilizer, regardless of the nitrogen supply from the substrate. The results are discussed with regard to hypotheses concerning the impact of nitrogen oxides on forest ecosystems.