Carbohydrate depletion during exercise was measured in the liver, in the three different types of skeletal muscle, and in the blood of exercise-trained and untrained rats. The acute exercise test consisted of 45 min of treadmill running of progressively increasing intensity. The training program consisted of 6 hrs of swimming per day, 5 days per week for 14 weeks; the training induced an increase of approximately 35 percent in the respiratory capacity of gastrocnemius muscle, and a 14 percent incrase in heart weight. Glycogen stores in fast-twitch red, fast-twitch white, and slow-twitch red types of skeletal muscle, were depleted significantly more slowly in the trained than in the untrained animals during the treadmill exercise test. Resting glycogen stores in the liver were higher and were depleted more slowly during exercise in the trained than the untrained animals. Blood lactate concentration was significantly lower in the trained than in the untrained rats at the end of the exercise test. These results provide evidence that endurance exercise training induces adaptation which protect against the depletion of glycogen from the liver and from the tree types of skeletal muscle during prolonged exercise.