This study was undertaken to investigate the effect of a combination of physical exercise and caloric restriction on weight loss, endurance capacity, and certain metabolic and biochemical parameters in middle-aged overweight subjects, two males and one female, during and also after a 30-day reduction period. The reducing diet provided 1600 Calories for the males and 1300 Calories for the female; the diets contained approximately 90 and 80gm protein and 90 and 80gm fat with a high ratio of polyunsaturated fatty acids, respectively. The exercise program consisted of a 2-hr walking on the treadmill at the speed of 80m/min. and at the inclination of 8.75%, four days a week. The female subject did not participate in the exercise program.The weight loss in the males was 4.5kg and in the female subject 2.2kg during the 30-day reduction period. The weight loss seen in the males could be divided into two phases. The measurements of body circumferences, skinfold thickness and nitrogen balance presented evidences that a rapid loss in the first 4-days of the experiment was principally caused by water loss and another rapid loss seen in the last week primarily came from loss of body fat, accompanied with a loss of lean body tissues. The weight was restored by 0.5 to 1.5kg for 30 days after realimentation. The daily 2-hr walking exercise which corresponded to 60% of Vo2 max(cc/min.) resulted in an increased Vo2 max(cc/min.)by 20 to 25% but did not prevent the lowering of the basal metabolic rate and development of deficiency anemia under caloric restriction. The activities of fasting serum GOT, GPT and creatine phosphokinase did not alter throughout the entire study. Urinary 17-KS which highly exceeded the normal level before the weight loss seen in three overweight subjects markedly declined during the reduction period. Urinary 17-OHCS rose slightly during the first few days, followed by a decline below the initial values, and urinary vanilmandelic acid remained unchanged during the study. Two male subjects fell into a state of nitrogen deficit but the female subject showed less negative nitrogen balance under caloric restriction. Urinary excretion of creatinine decreased significantly in one of the male subjects who showed a pronounced depression in the basal metabolic rate. The excretion of urinary uric acid was found to be depressed in all subjects on low calorie, high fat diets. Serum cholesterol and triglycerides showed a sharp decline within the first 7-days and thereafter were maintained at low levels throughout the reduction period. Realimentation, however, resulted in a rapid rise of serum lipid levels towards the individual initial values.It appears that a combination of regular dynamic exercise and caloric restriction act together for to cause a greater weight loss, better control of water retention, a marked increase of endurance capacity and a more pronounced reduction of serum cholesterol and triglyceride levels without causing significant damage to both liver and muscle tissues. Regular exercise under caloric restriction, however, can not act in preventing depression of the basal metabolic rate and the development of deficiency anemia.