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Abstract: Automatic regulation of tidal volume (V T) maintains CO2 homeostasis when spontaneous respiratory rhythm is replaced with a cortically triggered rhythm. We examined whether automatic regulation of respiratory frequency (f R) could maintain CO2 homeostasis at rest if the V T is cortically designated in experiments performed in 21 conscious humans. First, volitionally controlled f R at levels lower than baseline resulted in a larger V T, maintaining end-tidal CO2 fraction constant at eupneic levels. However, when f R was volitionally controlled at levels higher than baseline, end-tidal CO2 fraction decreased unexpectedly. Next, when the V T was volitionally constrained but f r was freely chosen, end-tidal CO2 fraction decreased. The present study revealed some limitations in the control of CO2 homeostasis by automatic regulation of f R, probably because respiratory rhythm is susceptible to non-metabolic factors. This study also showed the importance of automatic regulation of V T in maintaining CO2 homeostasis at rest. Nevertheless, automatic regulation of V T was incomplete when f R was volitionally imposed at high levels. [Copyright &y& Elsevier]