The effects of aluminum on growth, respiratory activities and on some enzymes associated to reactive oxygen species (ROS) elimination were examined in roots of two corn seedlings cultivated in aerated nutrient solution, in a susceptible (BR 106) and in a tolerant cultivar (BR 206). Aluminum (50 to 200 µM) decreased root dry mass only in the tolerant cultivar. Length of main root decreased in both cultivars, mainly in the susceptible one, at aluminum concentrations of 50 and 100 µM. Only in the tolerant cultivar aluminum induced a decrease in the leak of electrolytes. The susceptible cultivar, on the other hand, showed a significant increase in lipid peroxidation. Aluminum stimulated higher activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX), only in the tolerant cultivar. No difference was observed in the activities of catalase (CAT), peroxidases (POX) and glutathione reductase (GR) in both cultivars. Ascorbate content was not altered, but the ascorbate redox state (ratio ascorbate/dehydroascorbate) increased in the tolerant cultivar. Respiratory activities were evaluated in Percoll-purified mitochondria that exhibited similar respiratory control ratios in both cultivars. In absence of aluminum, respiratory oxygen consumption was higher in the tolerant cultivar, both in states 3 and 4. These respiratory activities were decreased by aluminum in the susceptible cultivar, but were increased in the tolerant one. ADP/O ratios were decreased by aluminum, in both cultivars to a similar extent. Cytochrome c pathway capacity was not altered by aluminum in the tolerant cultivar, but decreased in the susceptible one. Without aluminum, the susceptible cultivar showed a higher alternative oxidase (AOX) capacity than the tolerant one. Aluminum promoted a decrease in AOX capacity in the susceptible cultivar, the opposite response being observed in the tolerant one. In both cultivars aluminum induced an increase above 120% in the residual oxygen consumption. Activity of the plant uncoupling mitochondrial protein (PUMP) was decreased by aluminum in the susceptible cultivar and enhanced in the tolerant one. These results suggest that the tolerant cultivar possesses a more efficient enzymatic system for ROS removal than the sensitive one. Furthermore, the partial mitochondrial uncoupling, resulting from higher AOX and PUMP activities should have contributed to higher aluminum stress tolerance showed by cultivar BR 206.