The present study was intended to compare cardio-respiratory responses to the maximal exercise on bicycle with those on treadmill. The progressive loading method was adopted for both bicycle exercise and treadmill running (resistance for bicycle and speed for treadmill). The subjects were 28 well-trained middle and long distance male runners. aged 18 to 25 years. Douglas bags and Scholander's apparatus were used to determine the maximal oxygen uptake. The modified Filley's steady state method was adopted for the determination of pulmonary diffusing capacity, and CO_2 rebreathing method was for the determination of cardiac output. The results obtained in this study were summarized as follows; 1)The maximal oxygen uptake (3.93±0.47l/min) obtained by treadmill running was significantly greater than that (3.36±0.50l/min) by bicycle exercise. 2)The larger maximal oxygen uptake by treadmill running was due to the higher pulmonary diffusing capacity, ventilation, heart rate and / or arterial-venous oxygen difference than those by bicycle exercise.