The effects of hyberbaric nitrogen on the responses of ventilation and central inspiratory activity (CIA) to progressive hypercapnia were studied in eight subjects rebreathing a) O2 at an ambient pressure of 1.3 bar (control), and b) air at 6.1 bar (PO2 = 1.3 bar, PN2 = 4.8 bar). Inspiratory occlusion pressure (P0.1), pulmonary ventilation, and end-tidal PCO2 were used for the computation of individual CIA and ventilatory CO2 response curves. Increasing the inspired PN2 to 4.8 bar caused, on the average, a 40% increase of P0.1 at PCO2 = 50 Torr, whereas the slope of the ventilatory CO2 response curve was reduced by 39%. It was concluded that, at raised air and nitrogen pressures, CIA is increased, although not sufficiently to prevent a reduction of ventilation brought about by the increased gas density and consequent increase in airway resistance. The increased airway resistance is thought to be responsible for the increase in CIA by causing a reflex stimulation of the respiratory centers.