We tested the hypothesis that exposure to eccentric (lengthening) contractions results in greater damage and more prolonged recovery in aged rat muscle (32 months) than in adult muscle (6 months), and that the adaptation usually associated with a single exposure to eccentric exercise is reduced in the aged muscle. Experiments were performed using a new rat model for aging studies. Fisher 344/Brown Norway F1 Hybrid. An ankle flexor, the tibialis anterior (TA), was subjected to a series of 24 eccentric contractions in situ and contractile function was assessed 1, 2, 5 and 14 days following. Eccentric exercise produced a similar reduction in maximum specific twitch and tetanic tension in the aged and adult muscles at 1 and 2 days postexercise. Adult muscles recovered by 5 days, while aged TA remained significantly impaired. Aged TA was fully restored by 14 days. Exercise adaptation was tested by subjecting the TA to a second exercise 14 days following the first. Contractile function was determined 2 days following the second exercise. Adult TA maintained its pre-exercise specific force following the second exercise, while aged TA again showed a significant reduction. Thus, a single exposure to eccentric exercise produced complete adaptation in the adult TA, but not in the aged muscles.