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The participation of the antioxidant system in the drought tolerance of wheat cultivars (Triticum aestivum L.) was studied under field and in vitro conditions. Under field conditions, drought tolerance was evaluated by the capacity to maintain the grain yield under drought, which was higher in cvv. Elite and La Paz than in the sensitive cvv. Oasis and Cruz Alta. Tolerant cultivars showed lower relative water content (RWC) and lower above-ground vegetative biomass than sensitive cultivars. Field assays did not show a clear correlation between water-stress tolerance and antioxidant system behaviour. However, when leaves of cvv. with contrasting drought tolerance were subjected to osmotic stress in vitro, clear differences in the antioxidant system activity and oxidative damage between cvv. were observed. In the tolerant cultivar Elite, it was possible to observe an increase in ascorbate peroxidase (APX), superoxide dismutase (SOD) and glutathione reductase (GR) activities, a higher glutathione (GSH) and ascorbate content and less oxidative damage than in the sensitive cultivar Oasis, which showed no changes or only slight decreases in the enzyme activities. These results indicate that water stress tolerance is in part associated with the antioxidant system activity, and suggest that the behaviour of the antioxidant system in vitro assays can be used as an early selection tool.