Twelve-day-old soybean plants were exposed to atmospheric NO2 (0.3 μM1 1−1) and simultaneously supplied, via the roots, with 5 mM or 1 mM of NaNO3 or NH4C1. After exposure for 7 d, the amount of NO2 absorbed per plant was greater in plants supplied with nitrate than in plants supplied with the same concentration of ammonium. The NO2 AR (absorption rate) decreased with increasing exposure time. At the beginning of exposure, the NO2 AR for all plants was c. 12 mg NO2 h−1 m−2μ1−11. On the day 7 of exposure, the NO2 AR declined to 8.46, 8.97, 8.27, and 9.04 mg NO2 h−1 m−2μ1 1−1 1 for plants receiving 1 mM ammonium, 1 mM nitrate, 5 mM ammonium, and 5 mM nitrate respectively. The plants supplied with nitrate had a higher concentration of leaf nitrate and a higher pH than those supplied with the equivalent concentration of ammonium. These results suggest that the NO2absorption rate might be attenuated by the accumulation of H+ produced from N uptake and assimilation.