Body CO2 storage capacity was measured in nine subjects at two levels of exercise, approximating 30 and 60% VO2max, by measuring the excess CO2 output associated with hyperventilation at constant end-tidal PCO2, and the change in mixed venous PCO2 (PVCO2) measured by rebreathing. CO2 output was measured during 20-s periods and monitored throughout the procedure; hyperventilation was continued until CO2 output had returned to control values. Washout of CO2 was more rapid than previously found at rest, 90% of the change following an increase in ventilation occurring within 4 min. CO2 storage capacity was 1.83 +/- 0.552 (SD) ml-kg--1.Torr--1 at the lower power output and 1.19 +/- 0.490 ml-kg--1.Torr--1 at the higher power output. The CO2 storage capacity was inversely related to PVCO2. It was concluded that the body's capacity to store CO2 decreases with increasing PVCO2; this may be one factor leading to the progressive increase in pulmonary CO2 output at high levels of exercise.