The technique of intragastric infusion of calories was used to examine the energetics and metabolic response to fasting, feeding and overfeeding states in adult, sedentary subjects. All subjects were studied before and after participating in a 12-week aerobic exercise training program that increased maximum aerobic capacity by an average of 15.6 percent. Body weight did not change during the exercise program, but there was a slight and statistically significant increase in fat-free mass and a significant decrease in fat mass. The exercise program did not affect food intake (as determined from food records). When subjects were studied before exercise training, there was no increase in energy expenditure above fasting levels during continuous infusion of calories at a rate near the subject's maintenance energy requirements. When infusion rate was increased to twice maintenance, energy expenditure increased significantly and produced a measurable thermic effect of food. The changes in glucose, insulin and free fatty acids did not parallel the changes in energy expenditure, but changes in lactate levels did. Moreover, there were some gender differences in the pattern of substrate and hormone changes with changes in feeding state. Exercise training did not alter either energy expenditure or the pattern of changes in substrates and hormones with changes in feeding state. In summary, the exercise program led to changes in body composition without measurable changes in either food intake (assessed from diet records) or energy expenditure (assessed during fasting and continuous infusion of calories). The change in body composition must have been due to either (1) the energy cost of the exercise itself, or (2) changes in the substrates used for oxidation, either during or following exercise bouts. We conclude that exercise, unlike food restriction, can lead to changes in body composition without altering body weight or metabolic rate, and can contribute to the acquisition and maintenance of a desirable body weight and composition.