The effect of an increase in respiratory dead space on the ventilation during exercise was studied in 4 normal subjects breathing through a mixing chamber of variable volume at one or more work loads. The resulting increases in ventilation and alveolar Pco₂ were dependent on the work load and the volume of added dead space. The magnitude of the changes were related to the subject’s ventilatory response to CO₂, as measured at rest. The level of exercise, CO₂ output, or volume of dead space did not appear to have independent effects on the ventilatory response.