Dynamic characteristics of ventilation, cardiac output, and gas exchange in response to passive limb movements were studied in four healthy men in an upright position. Passive exercise was performed on a motor-driven bicycle ergometer, of which pedaling rate was varied from control (30 rpm) to stimulus (90 rpm) level in a stepwise fashion. Stroke volume (SV), heart rate (HR), and cardiac output (Q) were determined continuously during the exercise by using an automated impedance cardiograph. Minute ventilation (VE), respiratory frequency (f), tidal volume (VT), oxygen consumption (VO2), carbon dioxide output (VCO2), end-tidal pressure of oxygen and carbon dioxide (PETO2 and PETCO2), and the gas exchange ratio (R) were also determined at each breath. When the pedaling rate was increased, Q and VE rose in excess of metabolic need with a half response time of about 10 sec, and remained elevated for the duration of the stimulus. VO2 and VCO2 rose transiently, then recovered to the initial control level after a few min. PETCO2 remained at the control level for about one min, then decreased by 1 Torr. PETO2 and R rose transiently. These results suggest that hyperpnea during passive exercise is not induced by chemical stimuli to known chemoreceptors, but is due to reflexes mediated either by moving limbs or the right heart.