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The PaCOCO2 at the breaking point was found to be higher during exercise than during rest in four subjects. The breaking-point curve during exercise was elevated over the entire range of various oxygen concentrations studied, indicating that whatever the changes produced they are related to the contribution of CO2 to the urge to breathe and are independent of O2. At least part of the elevated PaCOCO2 may be accounted for by an overshoot of PaCOCO2 related to the rate of rise of Pco2. This was shown by increasing the rate of accumulation of CO2 by inhaling different O2-CO2 gas mixtures to simulate the increased CO2 production during exercise. However, certain calculations indicate that this is not the only difference between voluntary apnea during exercise as compared with that during rest. Submitted on June 30, 1961

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