Maximal oxygen uptake was measured in twenty-eight middle-aged men aged 45.0 (range: 40.3-50.7) years and fourteen middle-aged women aged 40.5 (range: 34.3-47.5) years by means of a treadmill test. The physique of subjects were:mean skinfold thickness at 4 sites, namely, back, arm, waist and abdomen was 7.59 mm in men and 14.5 mm in women; total body fat estimated from the mean skinfold thickness, 11.9% fat in men and 25.5% fat in women. The detailed observations are as follows: 1. Maximal oxygen uptake expressed on any reference unit in men was greater than that of women. The values of men to women were: 2.61 to 1.92l/min., 45.1 to 37.0 ml/min./kg, 51.8 to 46.5 ml/min./kg lean body mass (LBM), 15.7 to 12.6 ml/min./cm body height. 2. Maximal pulmonary ventilation in men was 80.2 l/min. and 63.5 l/min. in women. 3. There was a correlation of fairly high degree between maximal oxygen uptake and body weight, namely, r=0.56 in men and r=0.79 in women. The correlation between maximal oxygen uptake and lean body mass also was fairly high, namely, r=0.58 in men and r=0.69 in women. 4. The correlation between maximal pulmonary ventilation and body weight or lean body mass was r=0.51 and r=0.25 in middle-aged men and r=0.41 and r=0.19 in middle-aged women, respectively. 5. Maximal heart rate in men was 176.4 beat/min. and it was 183.9 beat/min. in women. 6. Maximal oxygen pulse in men was 14.6 ml/beat and 9.5 ml/beat in women. 7. Aerobic work capacity of korean middle-aged men and women maintained the same level as that of young men and women, respectively, as shown by maximal oxygen uptake expressed ml/min./kg or ml/min./kg lean body mass.