Idebenone effects on oxygen consumption in brain mitochondria, obtained from young and aged rats, were evaluated. Sixty rats (3 and 20 months of age) were treated for 3 months with 30 mg/kg of idebenone and compared to a placebo group. Brain mitochondria oxygen consumption rate was measured by polarographic techniques in basal (State 4), ADP-stimulated (State 3) and uncoupled conditions. When Complex I substrates (pyruvate + malate) were used, aged non-treated rats showed a significant increase in State 4 (175%) and uncoupled (152%) O(2)-uptake rate; no difference was found in State 3 respiration and in ADP/O(2) ratio. Idebenone was able to reverse these age-related effects probably acting on lipid peroxidation and the mitochondrial respiratory chain. No differences were found in mitochondrial enzymatic activities.