Mitochondrial DNA (mtDNA) mutations appear to be associated with a wide spectrum of human disorders and proposed to be a potential contributor of aging. However, in an age-dependent increase of the common 4977 bp deletion of human mtDNA still many unanswered questions remain. Comparing mtDNA copy levels in different tissues revealed that cardiac muscle had the highest, while the cortex cerebelli showed the lowest copy number of mtDNA in every donor. Intriguingly, mtDNA copy number showed no changes during aging. In heart tissue, the amount of 4977 bp mtDNA deletion increased in an age-dependent manner showing significant differences at the age of 40 years and older (p<0.005). In vitro studies analyzing human normal cells transfected with telomerase (BJ-T) revealed that oxidative stress (OS)--a well accepted promoter of aging--induced 4977 bp deletion and point mutations as demonstrated by real-time PCR and DHPLC analysis. Interestingly, OS induced apoptosis only in transformed human fibroblasts by activation of the intrinsic (mitochondrial-mediated) signalling pathway as indicated by morphological damage of mitochondria, DNA laddering and increase of the Bax/Bcl-2 ratio. In conclusion, in heart tissue, the amount of the 4977 bp deletion increased in an age-dependent manner and it was more detectable after the 4th decade of life, although there was some scatter in the data. Since, apoptosis was induced by the mitochondria-mediated pathway only in transformed cells, the role for apoptosis in normal tissue of the aging heart remains unclear.