Boluses of 133Xenon entrained in oscillating flow during high frequency oscillation (HFO) were used to study the effects of lung volume on the distribution of gas to topographical lung regions. The distribution of the tracer was measured with scintillation detectors positioned 5, 15 and 25 cm below each lung apex of subjects seated in a plethysmograph, while they maintained a patent glottis and constant breathhold. An index of HFO gas transport was obtained by measuring the 'wash-in' time for Xenon count rates to attain 0.25-0.75 of their final magnitude within each region. The distribution of gas was determined at 5 Hz with similar oscillatory volumes at FRC + 1 L, FRC, FRC - 1 L, and RV. Above FRC wash-in times were similar from apex to base. From FRC to RV, wash-in times at the bases became progressively longer. The results indicate that the regional distribution of HFO gas transport is dependent on lung volume.