The effect of different agents on inner-mitochondrial-membrane permeabilization and lipoperoxidation induced by Ca 2+ and the pyridine-nucleotide oxidant t-butylhydroperoxide or inorganic phosphate was investigated. Comparing the protection conferred by ADP, a substrate of the ADP/ATP carrier, dithiothreitol, a disulfide reductant and butylhydroxytoluene, a radical scavenger, it was found that ADP was always the most effective against mitochondrial damage, when present in the incubation medium from the beginning. Moreover, carboxyatractyloside, a specific inhibitor of the ADP/ATP carrier, abolished completely the protective effect of ADP on both the lipoperoxidation and mitochondrial swelling processes. Experiments where deenergized mitochondria were previously incubated with Ca 2+ showed a decrease in the content of active ADP/ATP carrier, indicating a direct involvement of this protein in the formation of a non-specific Ca 2+-dependent pore. Our results also eliminate the possibility of an attack of oxygen radicals on lipids or proteins of the mitochondrial membrane as the primary event triggering the permeability transition of the inner mitochondrial membrane.2153595600