Five male subjects exercised on a cycle ergometer (100 W) for 8 min; circulation to the legs was occluded by cuffs during the first 2 and last 2 min. Ventilation (VE), oxygen intake (VO2), and carbon dioxide output (VCO2) were measured breath by breath. Repeat studies were used to follow arterial PCO2 (PaCO2) and rebreathing mixed venous PCO2 (PVCO2). The results were compared to studies without cuffing, but which were otherwise identical. The initial period of cuffing was associated with marked hyperpnea, high VE/VCO2 ratio, and low PaCO2 and PVCO2. Following release of occlusion at the end of the first 2 min, there was an immediate fall in VE, followed by an increase after an average of 12 s. VE/VCO2 fell and end-tidal PCO2 rose after 4-5 s and reached control values after 12 s. Studies during rebreathing established that CO2 reached the lungs from the legs 4-5 s after release of occlusion, and control PVCO2 was reached after 12 s. Repeated occlusion for the final 2 min of exercise was associated with hyperpnea of similar degree to the initial occlusion. An identical study performed in a patient with absent ventilatory response to CO2 and reduced ventilatory response to exercise showed normal hyperventilatory response to cuffing but did not show an increase in ventilation associated with the arrival of CO2 in the lungs, following release of occlusion. The studies confirmed the importance of CO2 in mediating rapid changes in ventilation during exercise.