The non-reversing gas flow pattern in the avian lung has been attributed to 'aerodynamic valves'. A fundamental property of all aerodynamic valves is their dependence on inertial forces in the gas stream: sufficient reduction of inertial forces will cause aerodynamic valves to fail. If valving in the avian lung is aerodynamic, it should fail when gas stream momentum is reduced. We tested the dependence of the inspiratory valves in the goose lung on gas density and gas flow velocity. A bolus of tracer gas was placed in the tracheal cannula during an end-expiratory pause. Tracer gas appearance in a cranial air sac during the following inspiration and pause was used to deduce failure of the 'inspiratory valve' in cyclically ventilated geese. Little or no tracer entered the sac under control conditions, which approximated resting breathing, indicating highly effective valving. Lower flow rate or lower gas density caused increased tracer appearance, indicating valve failure. These results demonstrate the importance of gas inertial forces to normal valve function, and are direct evidence for the aerodynamic nature of the avian inspiratory valve.