Regular physical exercise retards a number of age-associated disorders, in spite of the paradox that free radical generation is significantly enhanced with exercise. Eight weeks of treadmill running resulted in nearly a 40% increase in maximal oxygen uptake in both middle-aged (20-month-old) and aged (30-month-old) rats. The age-associated increase in 8-hydroxy-2'-deoxyguanosine (8-OHdG) content was significantly attenuated in gastrocnemius muscle by exercise. The 8-OHdG repair, as measured by the excision of 32P-labeled damaged oligonucleotide, increased in muscle of exercising animals. The reactive carbonyl derivatives (RCD) of proteins did not increase with aging. However, when the muscle homogenate was exposed to a mixture of 1 mM iron sulfate and 50 mM ascorbic acid, the muscle of old control animals accumulated more RCD than that of the trained or adult groups. The chymotrypsin-like activity of proteasome complex increased in muscle of old trained rats. We suggest that regular exercise-induced adaptation attenuates the age-associated increase in 8-OHdG levels, and increases the activity of DNA repair and resistance against oxidative stress in proteins.