The myoglobin concentrations of gastrocnemius muscles of 2-, 5-, 9-, and 25-month-old sedentary, 25-month-old weight-restricted, and 25-month-old endurance-trained (treadmill running) Sprague-Dawley male rats were determined. The concentration of myoglobin was greater in the 5-month-old than in the 2-month-old rat and was lower in the 9- than in the 5-month-old rat. The gastrocnemius myoglobin concentration was the same for 9- and 25-month-old animals. Four months of endurance training, starting at 21 months of age, resulted in gastrocnemius myoglobin concentrations greater than either that of the 25-month-old weight-matched, calorie-restricted control or the 25-month-old sedentary control groups. The gastrocnemius myoglobin concentration of the elderly trained group was intermediate between that of the 5- and 9-month-old animals. These results are consistent with the known loss of oxidative capacity of skeletal muscle with age and its restoration with endurance exercise.