In 11 healthy subjects the effect of progressive hypoxia on pulmonary ventilation at various alveolar carbon dioxide pressures was studied. A rebreathing technique was used to produce hypoxia, CO2 was held constant and oxygen saturation was taken as the independent variable. We found a linear relationship between ventilation and falls in oxygen saturation when Pco2 was held at the resting mixed venous, end-tidal, or any intermediate level. Within this range of Pco2, a family of ventilation-So2 response curves was obtained for each subject. The effect of altering the isocapnic level was to change the slope and position of the ventilation-So2 response curve, the amount by which the slope changed being related to the slope for that subject at their mixed venous Pco2.