A particular mitochondrial DNA (mtDNA) deletion, the so-called "common deletion", accumulates progressively with age in human post-mitotic cells. We investigated the presence of age-related mtDNA deletions in mouse heart and, according to the free-radical theory of aging, the potential involvement of reactive oxygen species (ROS). Hearts from young, adult and old male Balb/c mice were homogenized and centrifuged in order to discard nuclear DNA. The supernatant was then utilized to prepare mtDNA by SDS-proteinase K digestion. The presence of a mtDNA4236 deletion was estimated by PCR analysis, by separating the amplificated segment on agarose gel. The incidence of the mtDNA4236 deletion was 16%, 28% and 78% in young, adult and old mice, respectively. 8-hydroxy-2'-deoxyguanosine (8-OH-dG), a marker of DNA oxidation, was also determined by HPLC-electrochemical analysis. 8-OH-dG was not detectable in young mice, while its concentrations (moles 8-OH-dG/10(6) moles dG; mean +/- SD) were 59.0 +/- 1.41 and 31.0 +/- 4.24 (p < 0.02) in adult and old mice, respectively. These data indicate that a mtDNA4236 deletion is progressively associated with aging in mouse hearts, and that oxidative damage to mtDNA is greater in middle-aged than senescent animals.