Subpleural concentrations of He and SF6 were measured during multiple-breath washouts from isolated dog lungs. Tidal volume, inspiratory flow, and frequency were in the normal range of canine ventilation. For each gas, there was a local minimum in concentration during inspiration (C(insp)) and a local maximum in concentration during exhalation (C(exp)). SF6 exhibited a deeper inspiratory trough than He for each breath of every washout. For large tidal volumes (10-20 ml/kg), C(exp) approximated a single exponential decay and He was cleared more rapidly than SF6. For small tidal volumes (2.5 ml/kg), C(exp) was multiexponential and SF6 was cleared more rapidly than He. C(insp)/C(exp) (a measure of the depth of the inspiratory trough) and the kinetics of C(exp) decay were determined for washouts using a tidal volume of 10 and 20 ml/kg and different inspiratory flows. Under all conditions, an increase of inspiratory flow resulted in a deeper inspiratory trough for both He and SF6. For washouts using 10 ml/kg and 60 breaths/min, an increase of inspiratory flow increased the clearance of both gases. In washouts using lower ventilatory frequencies, gas clearance was independent of inspiratory flow. These findings are contrary to predictions of contemporary models of convection and diffusion in the lung. This study suggests that convective axial mixing and radial diffusion in the airways are important determinants of pulmonary gas transport.