A two-compartment mechanical model of the lungs was constructed with two parallel peripheral and collapsible bronchi in series with one central and collapsible trachea. Maximal expiratory flow-volume (MEFV) curves similar to those obtained in most dogs and in some humans could be produced: a peak followed by a gently sloping plateau ending in a knee, where flow suddenly fell to a much smaller value approaching zero rather slowly over the last 25 to 50% of the expired vital capacity. It was shown that flow before the knee was limited in the trachea, and after the knee it was limited in the bronchi. Two patterns of changes in the configuration of the MEFV curve could be observed. Pattern of changes affecting the central airway, at a given volume, maximal flow during the first part of the expiration (i.e., before the knee) is decreased; the knee occurs at a lower lung volume; the flow at the beginning of the knee is decreased. This pattern was observed with the following interventions: decreased cross-sectional area of the trachea (partial obstruction); decreased axial tension of the trachea; and, increased frictional loss between the trachea and the bronchi. Pattern of changes affecting the airways in the periphery: the knee occurs at a higher lung volume; at a given volume, flow after the knee becomes smaller; the absolute flow at the start of the knee is almost unchanged. This pattern was observed with the following interventions: decreased cross-sectional area of the peripheral airways (partial obstruction); increased frictional loss upstream to the peripheral airways; and, decreased elastic recoil pressure.(ABSTRACT TRUNCATED AT 250 WORDS)