Plants of Zea mays L., Hordeum vulgare L. and Arabidopsis thaliana (L.) Heynh. were grown at different concentrations of nitrogen, as NH4NO3, and the antioxidant enzyme activities and quantities in the roots, were studied. Maize plants were grown at 1 and 10 mM N for 32 d after germination and their roots were analysed by native PAGE for the isoenzymic profiles of glutathione reductase (GR; EC 220.127.116.11), catalase (CAT; EC 18.104.22.168) and superoxide dismutase (SOD; EC 22.214.171.124) and by spectrophotometric assays for total activity of CAT and GR. Barley and A. thaliana plants were grown at five N concentrations (0.25, 0.5, 1, 10 and 20 mM) for 27 and 26 d, respectively, and the isoenzymic profile of the three enzymes were analysed in their roots. The number of GR isoforms in the three plant species was increased at high N supply compared with low N. In addition, the CAT and GR activities detected in the roots were increased in plants grown at higher N. The increase in activity and number of bands of GR at high N in the roots of all three plants studied was not accompanied by a change in activity or the number of SOD isoforms. The possibility that the application of high N promotes the formation of reactive oxygen species by stimulating an increased rate of growth is discussed.