Foliar absorption of nitrogen dioxide in barley (Hordeum vulgare L.) exposed to mean NO2 concentrations of 4 and 37 ppb was studied in open-top chambers. The possible compensation for insufficient supply of nitrogen in the growth medium by increased uptake of NO2 from the air was investigated by growing plants with sub-optimal and optimal N supply. The amount of NO, derived from fumigation was estimated using the 15N dilution technique, in which the nitrogen derived from the growth medium was labelled with 15N. The absorption of NO2-N from polluted air contributed 5–6% of the total N. An additional 3–5% was derived from other N sources, probably also of gaseous origin. Thus, total absorption from gaseous sources in the NO2-fumigated plants was estimated to be approximately 10% of total N. The amount absorbed from aerial sources was proportional to the top weight. The amount of NO2 absorbed was highest in the plants with optimal N supply. However, in plants grown with sub-optimal N, there was a relatively higher contribution to total N from the NO2-N.