The effects of different intensities of cycling exercise on airway resistance (Raw), thoracic gas volume (Vtg), and specific airway conductance (sGaw) were studied in 7 normal male subjects using a body plethysmograph. A pressure type body plethysmograph having the internal volume of about 1,100 liters was made and a remote controlled bicycle ergometer was equipped in the plethysmograph to be able to carry out exercise in the box. Exercise of three different loads (300 kpm/min, 600 kpm/min, and 900 kpm/min) was performed at a pedaling rate of 50 rpm for five min each. After base line measurement at rest, Raw, Vtg, and sGaw were measured intermittently during exercise and recovery period. The results obtained were as follows: 1. Raw decreased during exercise to 70-80% of the base line. 2. The decrease of Raw was larger at the first minute of the severe exercise than in the light or moderate exercises and became smaller with time of exercise. This trend was also found in the light and moderate exercises with indistinct degree. 3. Raw which decreased during exercise returned to the base line level at the early recovery period and thereafter maintained a little lower value than the base line. 4. Vtg increased during exercise and the larger the intesity of exercise the longer the time returning to the base line after exercise. 5. The increase of sGaw during exercise amounted to 111-137% of the base line at the early moment and became smaller with time of exercise. This tendency was more distinct in the severe exercise than in the light or moderate exercises. The discussion of these results suggests that the changes in Raw and sGaw during and after exercise were basically resulted from the antagonistic action between the change in autonomic balance and the augmented ventilation. Also the unproportionate relationship between the degree of changes in Raw and sGaw and the intensity of exercises can be interpretted by above mentioned mechanism.