To assess the role of direct alveolar ventilation during high-frequency ventilation, we studied convective gas mixing during high-frequency oscillation with tidal volumes close to the dead space volume in a simple physical model. A main conduit representing a large airway was connected with a rigid sphere (V = 77, 517, and 1,719 cm3) by a small circular tube (d = 0.3 and 0.5 cm; L = 5, 10, and 20 cm). The efficiency of sinusoidal oscillations (f = 5, 20, and 40 Hz) applied at one end of the main conduit was assessed from the washout of a CO2 mixture from the sphere; to flush CO2 from the main fluid line, a constant flow of air was used. The decay in CO2 concentration measured in the sphere was exponential and therefore characterized by a measured time constant (tau m). Taking the small tube volume as the ventilatory dead space (VD), an effective tidal volume (VT*) was computed from tau m and compared with the tidal volume (VT) obtained separately from the pressure variation in the sphere. The discrepancy between these two tidal volumes has been found to be uniquely dependent on the ratio VT/VD within the range of VT/VD studied (0.5-2.2). For VT/VD less than 1.2, VT* was larger than VT, indicating that the conventional concept of alveolar ventilation does not apply. From the partition of the oscillatory flow in the small tube into two regions, the core and the unsteady boundary layer, we theoretically computed the proportions of the sinusoidal flow (or tidal volume) and the dead space for each region.(ABSTRACT TRUNCATED AT 250 WORDS)