The effects of salt stress on growth and respiratory activities were examined in roots of maize seedlings cultivated for ten days in a Hoagland nutrient hydroponic solution. Root dry mass was decreased by NaCl (100 μM) in all cultivars studied (AGN 3150, BR 106, BR 201, BR 206 and SHS 4040). Cultivar AGN 3150 showed the shortest decrease in root growth (22%) and was considered as salt-tolerant, contrasting with cultivar BR 106 considered as salt-sensitive presenting 48% reduction in growth. Respiratory activities were evaluated in Percoll-purified mitochondria isolated from roots of the latter two cultivars. In absence of NaCl, respiratory oxygen consumption was similar in the two selected cultivars, both in states 3 and 4. Salt treatment decreased state 3 only in the sensitive cultivar. State 4 respiration, on the other hand, was enhanced by salt in the tolerant cultivar and decreased in the sensitive one. Salt decreased respiratory control ratio only in the tolerant cultivar and decreased ADP/O ratios in both similarly. Cytochrom c pathway capacity was not altered in the tolerant cultivar but was strongly decreased (50%) in the sensitive one. Alternative oxidase capacity was similar in both cultivars in control situation. Salt stress enhanced the alternative oxidase capacity about 92% in the tolerant cultivar while no significant difference was observed in the sensitive cultivar. Residual oxygen consumption was enhanced three fold by salt stress in the tolerant cultivar without significant differences in the sensitive one. Plant uncoupling mitochondrial protein (PUMP) activity, evaluated by means of a TPP⁺ electrode, was decreased by salt stress in the tolerant cultivar and enhanced in the sensitive one. Electrolyte leaking enhanced about 2.4 fold only in the sensitive cultivar. Lipid peroxidation, evaluated by malondialdehyde content was enhanced about 120% in the sensitive cultivar by salt treatment. These results are not clear enough to elucidate the role of mitochondria on the mechanisms that render the AGN 3150 cultivar a more tolerant do salt stress than BR 106 cultivar. Membrane damages observed as a consequence of salt treatment indicate possible oxidative stress and suggest the need for further investigation related to the enzymes involved in the metabolism of the reactive oxygen species.