The mitochondrial genome is essential for producing ATP (adenosine 5'-triphosphate) via oxidative phosphorylation. The gradual decline of mitochondrial function with age has long been postulated as a factor in aging. More recently, a variety of diseases have been related to molecular defects in human mitochondrial DNA. In both the cases of aging and disease, symptoms were generally neuromuscular, reflecting the tissues most dependent upon mitochondrial function. Also, in both cases novel features of mitochondrial genetics led to complex relations between genotype and phenotype. Little information is yet available about the role of environmental agents in these interactions.