Reactive oxygen species may contribute to exercise-induced skeletal muscle damage, and antioxidants may protect against such damage. This study examined the effectiveness of prophylactic supplementation with vitamins C and E on symptoms of muscle damage in a single blind, two-group study design. Twelve male volunteers were randomly assigned to either treatment or control groups. The treatment group received 500 mg of vitamin C and 1,200 IU of alpha-tocopherol daily and the control group received glucose placebo for 37 days. After 30 days of treatment, volunteers performed 300 maximal eccentric contractions of the knee extensor muscles of one leg. Maximal voluntary isometric contraction force and electrically evoked force at a frequency of 20 Hz and 50 Hz were recorded before and after exercise, and on days 1, 2 and 7 after exercise. Muscle soreness questionnaires were completed and muscle girth recorded at the same time points. Eccentric contractile torque and work during the bout declined significantly in both groups ( P<0.001), but this decline was smaller in the vitamin-supplemented group ( P<0.05). Maximal voluntary isometric contraction force and 20:50 Hz force ratio declined significantly after exercise in both groups ( P<0.01), but the decline was smaller in the treatment group on days 1 and 2 post-exercise ( P<0.05). Both groups experienced similar significant muscle soreness and swelling after exercise. These data suggest that prior supplementation with dietary antioxidants ameliorates muscle functional decrements subsequent to eccentric muscle contraction.