Cultures of water fern Azolla pinnata R. Br. exposed for 1 week to either 30, 50 or 80 nl l-1 O3 showed significant reductions in rates of growth and N2 fixation, and had fewer heterocysts. Although the levels of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activity were decreased by low concentrations of O3 exposures (30 or 50 nl l-1), significant increases in levels of the same enzymes were caused by higher concentrations of O3 (80 nl l-1). Increased levels of total protein, polyamines (putrescine and spermidine), and the xanthophyll-cycle precursor of abscisic acid (ABA), violaxanthin, were also found with higher levels of O3 (80 nl l-1). Levels of ABA itself were significantly increased by low level O3 fumigation (30 nl l-1) but significantly decreased by exposure to 80 nl l-1 O3. This may indicate that higher levels of atmospheric O3 inhibit the final stages of ABA biosynthesis from violaxanthin.