Mi to chon drial the ory of ag ing, a vari ant of free rad i cal the ory of ag ing, pro poses that ac cu mu la tion of dam age to mi to chon dria and mi to chondrial DNA (mtDNA) leads to ag ing of hu mans and an i mals. It has been sup ported by the ob ser va tion that mi to chon drial func tion de clines and mtDNA mu ta tion in creases in tis sue cells in an age-dependent man ner. Age-related im pair ment in the re spi ra tory en zymes not only de creases ATP syn the sis but also en hances pro duc tion of re ac tive ox y gen spe cies (ROS) through in creased elec tron leak age in the re spi ra tory chain. Human mtDNA, which is not pro tected by his tones and yet is ex posed to high lev els of ROS and free rad i cals in the ma trix of mi to chon dria, is sus cep ti ble to ox i da tive dam age and mu ta tion in tis sue cells. In the past de cade, more than one hun dred mtDNA mu ta tions have been found in pa tients with mi to chon drial dis ease, and some of them also oc cur in ag ing hu man tis sues. The in ci dence and abun dance of these mu tant mtDNAs are in creased with age, par tic u larly in tis sues with great demand for en ergy. On the other hand, re cent stud ies have re vealed that the abil ity of the hu man cell to cope with ox i da tive stress is com promised in ag ing. Com par a tive anal y sis of gene ex pres sion by microarray tech nol ogy has shown that a num ber of genes re lated to ox i da tive stress re sponse are al tered in ag ing an i mals. We dis cov ered that the tran scripts of early growth re sponse pro tein-1, growth ar rest and DNA dam age-inducible pro teins and glutathione S-transferase genes are increased in re sponse to ox i da tive stress in hu man skin fibroblasts. More over, the ac tiv i ties of Cu, Zn-SOD, catalase and glutathione peroxidase de crease with age, whereas Mn-SOD ac tiv ity in creases with age up to 65 years and slightly de clines there af ter in skin fibroblasts. Such an im bal ance in the func tion of an ti ox i dant en zymes may re sult in ex cess pro duc tion of dam ag ing ROS in the cell. This no tion is sup ported by the ob ser va tion that intracellular lev els of H2O2 and ox ida tive dam age to DNA and lipids are sig nif i cantly in creased with age of the fibroblast do nor. Fur ther more, the mi to chon drial pool of reduced glutathione de clines and DNA dam age is en hanced in ag ing tissues. Taken to gether, these ob ser va tions and our pre vi ous find ings that mtDNA mu ta tions and ox i da tive dam age are in creased in ag ing hu man tis sues sug gest that mi to chon drial the ory of ag ing is ma ture.