1. Rapid steady-state CO2 responses were determined in five normal adults at rest and at up to six levels of exercise by injecting pure CO2 at a constant flow into the inspiratory limb of a breathing circuit.

2. Ventilation (V) was measured with a dry gas meter and Pco2 at the mouth was recorded by a mass spectrometer. Mean alveolar Pco2 (Paco2) was taken as equal to end-tidal Pco2 at rest, and during exercise was derived graphically from the sloping alveolar plateaus. The accuracy of the latter method was checked in separate experiments against arterial Pco2 (Paco2).

3. The mean results showed a linear relationship between change in Paco2 and change in V for work loads ranging from rest to 75 W (r = 0.94–0.98). Above 75 W the response became concave down with an initial essentially isocapnic phase.

4. This suggests that during exercise there is a large increase in CO2 sensitivity about the control point.

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