The energy metabolism of ad lib.-fed adult male Wistar rats receiving daily running exercise (0.9 km/d; 8 degrees incline) on a motor-driven treadmill, over a period of 56 d, was compared with that of sedentary ad lib.-fed rats and sedentary restricted-fed rats of similar body-weight (approximately 420 g). 2. The metabolizable energy of the diet (Oxoid 41 B) was 11.44 +/- 0.05 kJ/g. This value was not affected by restricted feeding (70% ad lib.), exercise training or exercise itself. 3. Exercise-trained rats ate 5% more food than the sedentary ad lib.-fed rats but their equilibrium body-weight was 60 g lower than that of the latter group. 4. Resting metabolic rate, measured over 22 h in a respiration chamber was increased by 10% in exercise-trained animals. 5. Feeding increased energy expenditure (dietary-induced thermogenesis) and this effect was potentiated by performance of an exercise task. 6. Exercise-trained rats exhibited anticipatory rises in energy expenditure (approximately 40%) when placed on a stationary treadmill. 7. Treadmill work increased energy expenditure by a factor of 1.9-2.4. 8. The energy cost of the exercise, determined by respiration calorimetry was 66-80 J/g per km. These energy costs did not account for all the differences observed in food energy consumption of exercise-trained and sedentary rats of equal body-weight. 9. It is concluded that regular physical exercise increases energy expenditure by factors additional to the energy requirement directly related to the physical work. These factors include an increased resting metabolic rate in exercise-trained rats, increased dietary thermogenesis induced by exercise and anticipatory increases in energy metabolism during the period preceding exercise.
