DNA damage is considered of paramount importance in aging. Among causes of this damage, free radical attack, particularly from mitochondrial origin, is receiving special attention. If oxidative damage to DNA is involved in aging, long-lived animals (which age slowly) should show lower levels of markers of this kind of damage than short-lived ones. However, this possibility has not heretofore been investigated. In this study, steady-state levels of 8-oxo-7, 8-dihydro-2'-deoxyguanosine (8-oxodG) referred to deoxyguanosine (dG) were measured by high performance liquid chromatography (HPLC) in the mitochondrial (mtDNA) and nuclear (nDNA) DNA from the heart of eight and the brain of six mammalian species ranging in maximum life span (MLSP) from 3.5 to 46 years. Exactly the same digestion of DNA to deoxynucleosides and HPLC protocols was used for mtDNA and nDNA. Significantly higher (three- to ninefold) 8-oxodG/dG values were found in mtDNA than in nDNA in all the species studied in both tissues. 8-oxodG/dG in nDNA did not correlate with MLSP across species either in the heart (r=-0.68; P<0.06) or brain (r = 0.53; P<0.27). However, 8-oxodG/dG in mtDNA was inversely correlated with MLSP both in heart (r=-0.92; P<0.001) and brain (r=-0.88; P<0.016) tissues following the power function y = a(.)x(b), where y is 8-oxodG/dG and x is the MLSP. This agrees with the consistent observation that mitochondrial free radical generation is also lower in long-lived than in short-lived species. The results obtained agree with the notion that oxygen radicals of mitochondrial origin oxidatively damage mtDNA in a way related to the aging rate of each species.-Barja, G., Herrero, A. Oxidative damage to mitochondrial DNA is inversely related to maximum life span in the heart and brain of mammals.