It has been widely postulated that age-dependent changes in the mitochondrial genetic system may contribute to the human aging process. We recently reported unchanged specific activities of mitochondrial respiratory chain enzymes and a decrease in oxidation capacity of different substrates with aging, due, in part, to some confounding variables such as physical activity or tobacco consumption. The present study deals with age-related changes in muscle mtDNA structure and its biogenesis in humans. We found a low prevalence of mtDNA rearrangements with aging, only detected by PCR. The mtDNA content increased significantly with age (b = 0.0115, P < 0.0001). Also, an unchanged steady-state level of mitochondrial transcripts, a reduced transcription rate (P < 0.0001), and an increase in mitochondrial membrane lipid peroxidation (P < 0.0001) were observed in aging. These data demonstrate that minor structural mtDNA changes appear during the human aging process. By contrast, alterations in mitochondrial homeostasis ultimately producing modifications in mitochondrial biogenesis rates could play a role in the process of human senescence.