This study was conducted to observe the effects of endurance exercise training on antioxidant enzyme activity in the liver and gastrocnemius muscle of rats being fed dietary casein and soy protein. The respective influences of dietary casein and soy protein on the activity of antioxidant enzymes were also compared. Thirty-nine male Wistar rats, aged 3 weeks, were randomly assigned to six groups: a normal protein control group, a normal protein endurance training group, a casein protein control group, a casein protein endurance training group, a soy protein control group, and a soy protein endurance training group. The endurance exercise training groups were adapted to a treadmill for 2 weeks prior to the date the rats were forced to run for 60 min at 25 m/min, 5 days/week for 12 weeks. We found that antioxidant enzyme activity in the gastrocnemius muscle was neither effected by the dietary proteins (casein and soy protein) nor by the above endurance exercise training load. However, hepatic Cu,Zn-SOD activity increased significantly for the dietary casein and soy protein diet groups as compared with the normal protein diet group (P < 0.01). Furthermore, significant increases both in hepatic Cu,Zn-SOD activity in the normal protein group and hepatic GSHpx activity in the casein and soy protein groups were observed when rats were loaded with 25 m/min of endurance exercise training (P < 0.01). These results suggest that, under the above experimental conditions, a casein or soy protein diet increase hepatic Cu,Zn-SOD activity, while endurance exercise training is effective in increasing hepatic Cu;Zn-SOD activity on a normal protein diet and in increasing hepatic GSHpx activity for cysteine and methionine deficient diets.