When lipid peroxidation was induced in isolated mitochondria there was a marked increase in the 8-hydroxyguanine content of the nucleic acids extracted from these mitochondria. The elevation of 8-hydroxyguanine levels was associated with an extensive alteration of normal electrophoretic mobility of mitochondrial DNA. However, suppression of lipid peroxidation with alpha-tocopherol proportionally attenuated 8-hydroxyguanine production and limited the electrophoretic mobility change of mitochondrial DNA.