Exposures of sunflowers (Helianthus annuus L. var. Giganteus) to 15NO2, at concentrations between 5.0 and 72.9 ppb resulted in its uptake by the plants during illumination as well as during darkness. The NO2, flux increased linearly with increasing NO2, concentrations in both treatments. Evolution of NO2 by sunflowers was not observed. During illumination, measured and potential N Of uptake were equal, which indicates that the uptake of NO2 was only limited by stomatal resistance, whereas in darkness, an additional internal resistance to NO. cannot be excluded. After 15NO2, exposures, the highest proportions of the 15N tracer were fixed as protein nitrogen or as heterocyclic nitrogen compounds and glucosamine. Under both treatments there were remarkably high 15N enrichments in the fraction ‘soluble amino acid nitrogen’, indicating a rapid assimilation of the nitrogen derived from 15NO2, A linear relation was found between the δ15N values of nitrogen pools investigated and the 15NO2 concentrations after exposures in light as well as after night-time fumigation. Apparently, all the enzyme systems involved in the assimilation of the NO2 nitrogen must be available in sufficient amounts in plant tissues during exposures. Even after exposures in the dark, about 95% of the absorbed 15NO2, nitrogen was fixed in reduced organic nitrogen compounds, which indicates that the assimilation of atmospheric NO2, might not depend on the production of reduction equivalents by photosynthesis. The mechanism of NO, assimilation via nitrate/nitrite reductase within the exposed sunflowers is discussed.