Density of the gas inspired by 3 adult sitting males was changed by varying gas composition and/or ambient pressure, and the effects on respiratory resistance, maximum voluntary ventilation (MVV), and maximum flow were determined. When the same gas density was obtained by 2 different combinations of composition and pressure, there was no significant difference. The nonelastic pressure, Pnon-el, required to move a gas of density D at a flow rate V is given by the empirical equation Pnon-el = 0.40 (D + 0.70) V1.6. When gas density is 3 times that of air at sea level, MVV drops to 50% of control value. Maximum flow decreases similarly. Flow and MVV are limited by a combination of airway resistance and inherent limitations in the ventilatory pump (tissue resistance and time required for muscle shortening). With no gas in the airway, the intrinsic pump factors would limit the rate of change of lung volume to 14 1-sec-1.