The effect of external resistance on the relationship of expiratory air flow and intrathoracic and mouth pressure on subjects breathing gas of increased density was investigated. Five subjects breathing air performed multiple maximum forced expiratory maneuvers through orifices of various sizes at 1, 4, and 7 ATA in a standard double-lock compression chamber. Measurements of flow and pleural, mouthpiece, and transpulmonary pressures were made, and flow-volume and pressure-volume curves were constructed and analyzed at 75, 50, 35, and 25% of vital capacity. At each lung volume, maximum flow could be maintained until a certain orifice size was reached. This "limiting orifice" was 7.5--10 mm in diameter and did not change with lung volume or density. Under the conditions studied, this finding leads to the conclusion that the flow-limiting segment of the lungs behaved as a rigid orifice less than 10 mm in diameter. Orifices slightly larger than the limiting orifice increased pleural pressure, as expected, but transpulmonary pressure decreased while the flow remained stable, which indicates that airway compression may be lessened by increased intra-airway pressure.