Cultures of the water fern Azolla pinnata R, Br. exposed for 1 week to atmospheric NO2 (50, 100 or 200 nl l-1) induced additional levels of nitrate reductase (NaR) protein and nitrite reductase (NiR) activity. At low concentrations of NO2 (50 nl l-1), nitrate derived from NO2 provides an alternative N source for Azolla but does not affect rates of acetylene reduction. However, the symbiotic relationship between Azolla and its endosymbiont, Anabaena azollae is only affected adversely by high concentrations (100 and 200 nl l-1) of atmospheric NO2. The resultant decreases in rate of growth, nitrogen fixation, heterocyst formation, and overall nitrogen cycling are probably due to the additional accumulation of N products derived from higher levels of atmospheric NO2. Parallel increases in levels of polyamines suggest that Azolla partially alleviates these harmful effects by incorporating some of the extra NO2-induced N into polyamines.