The electron transport activities of various respiratory enzyme complexes in the muscle mitochondria of subjects of different ages without known mitochondriopathies were investigated. The results showed that the activity of cytochrome c oxidase declined with age more drastically than the activities of other respiratory enzyme complexes. NADH-cytochrome c reductase activity decreased mildly with age, whereas succinate cytochrome c reductase activity did not show significant age-dependent changes. Deletions in the muscle mitochondrial DNA (mtDNA) by use of PCR techniques were investigated. Three different age-dependent deletions in the muscle mtDNA of old individuals were found. These were identified to have sizes of 4,977 bp (8470 to 13446), 6,063 bp (7,842 to 13,904) and 7,436 bp (8,649 to 16,084). The 4,977 bp deleted mtDNA (dmtDNA) started to appear in the muscle mtDNA of subjects over 36 years of age and was found to exist in the muscle of more than 70% of the study subjects over 60 years of age. The 6,063 bp dmtDNA was detected in the muscle of the subjects above 25 years of age and was present in the muscle of about 91% of the individuals above 60 years old. However, the 7,436 bp deletion was less prevalent and was only seen in the muscle mtDNA of about 47.2% of the study subjects that were above 60 years. Using a quantitative PCR method, we found that the proportion of the 7,436 bp-deleted mtDNA increased with age, although with notable individual differences. Some of the subjects were found to have multiple deletions in their muscle mtDNAs, but some others carried only a specific type of deletion. The frequency of occurrence of multiple deletions in the muscle mtDNA was significantly increased with the age of the study subjects. The age-dependent increase in the proportions of various deleted mtDNA molecules may cause deleterious effects on the expression of mitochondrial genes in the muscle of old humans. This may account, at least in part, for the age-dependent decline of respiratory functions in the mitochondria of a broad spectrum of human tissues. We suggest that deletions of mtDNA are early molecular events that are associated with and contribute to the ageing processes of the human.