In the inhomogeneous lung, alveolar pressure is not the same in all the functional units at any given time. The meaning of airway resistance, measured from rate of flow and mean alveolar pressure at a given time, is considered theoretically. The system studied is a mechanical analog of 2 pulmonary pathways connected in parallel, each with its own linear flow-resistance and volume-elastic properties. In such a system, "apparent" airway resistance varies with flow distribution. When the 2 compartments have equal gas volumes, apparent resistance is linearly related to relative ventilation of 1 compartment. Variations of apparent resistance with time are large, but different according to the temporal pressure pattern applied to the whole system. Although local resistances are viscous and linear, apparent resistance is nonlinear as shown by pressure/ flow relationships. This apparent airway resistance does not have the same meaning as the total intrapulmonary resistance measured from graphical analysis of transpulmonary pressure.