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Mitochondrial respiratory chain complexes I and III have been shown to produce superoxide but the exact contribution and localization of individual sites have remained unclear. We approached this question investigating the effects of oxygen, substrates, inhibitors, and of the NAD⁺/NADH redox couple on H₂O₂ and superoxide production of isolated mitochondria from rat and human brain. Although rat brain mitochondria in the presence of glutamate+malate alone do generate only small amounts of H₂O₂ (0.04 ± 0.02 nmol H₂O₂/min/mg), a substantial production is observed after the addition of the complex I inhibitor rotenone (0.68 ± 0.25 nmol H₂O₂/min/mg) or in the presence of the respiratory substrate succinate alone (0.80 ± 0.27 nmol H₂O₂/min/mg). The maximal rate of H₂O₂ generation by respiratory chain complex III observed in the presence of antimycin A was considerably lower (0.14 ± 0.07 nmol H₂O₂ …