Gas transport during high-frequency oscillation (HFO) and high-frequency jet ventilation (HFJV) was compared in a simple lung model of human conducting airways. The delivery of gas to distal airways was assessed by measuring the dilution of 5% CO2, introduced separately into the model. Increasing frequency from 1.0 to only 1.7 Hz redistributed gas into straighter airway paths during HFO. Gas flow rates during HFO were sinusoidal, and CO2dilution at the most distal airway sampling site was a function of both mean and peak flow rates. CO2dilution increased as frequency increased, even though tidal volume (Vt) was constant. Peak flow increased as either Vt or frequency was increased (range 132 to 2167 ml/sec). During HFJV, flow approximated a square-wave function. Peak flows during HFJV were lower than during HFO, except at mean flows less than 167 ml/sec. Although CO2dilution at the most distal airway sampling site increased as Vt increased during HFJV, CO2dilution actually decreased as frequency was increased at any given Vt. Thus, in this model, gas transport in large airways differed during HFO or HFJV, and was related to peak as well as mean flows. Also, the distribution of gas within the airways changed as frequency increased.