In the phytotron experiment, the effect of elevated atmospheric CO2 (EC, 750 μmol mol-1) on the drought tolerance was studied in two winter varieties (Mv Mambo, tolerant; Mv Regiment, moderately tolerant) and in one spring variety of wheat (Lona, sensitive to drought). Changes in net photosynthetic rate (P N), stomatal conductance, transpiration, wateruse efficiency, effective quantum yield of photosystem II, and activities of glutathione reductase (GR), glutathione-Stransferase (GST), guaiacol peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were monitored during water withdrawal. Drought caused a faster decline of PN at EC, leading to the lower assimilation rates under severe drought compared with ambient CO2 (NC). In the sensitive variety, PN remained high for a longer period at EC. The growth at EC resulted in a more relaxed activation level of the antioxidant enzyme system in all three varieties, with very low activities of GR, GST, APX, and POD. The similar, low values were due to decreases in the varieties which had higher ambient values. A parallel increase of CAT was, however, recorded in two varieties. As the decline in PN was faster at EC under drought but there was no change in the rate of electron transport compared to NC values, a higher level of oxidative stress was induced. This triggered a more pronounced, general response in the antioxidant enzyme system at EC, leading to very high activities of APX, CAT, and GST in all three varieties. The results indicated that EC had generally favourable effects on the development and stress tolerance of plants, although bigger foliage made the plants more prone to the water loss. The relaxation of the defence mechanisms increased potentially the risk of damage due to the higher level of oxidative stress at EC under severe drought compared with NC., S. Bencze, Z. Bamberger, T. Janda, K. Balla, B. Varga, Z. Bedõ, O. Veisz., and Obsahuje bibliografii