1. Seated subjects stopped ventilation briefly at end expiration while a 5 ml bolus of 133Xe was injected close to the mouth. They then inspired air at different flow rates and the distribution of radioactivity in the lungs was measured with a scanning technique during a period of breath-holding at maximal inspiration.

2. In five normal subjects the dependent zones received a greater fraction of the 133Xe bolus than the apex during slow inspirations, but apical distribution exceeded basal for fast inspirations. The volume history of the lungs before the bolus injection had no effect on the slow/fast difference in four out of five subjects.

3. In five patients with clinical bronchitis but normal forced expired volume, dependent zone ventilation was much reduced on a slow inspiration compared with normals, but at fast flow rates the distribution was normal.

4. Insofar as the bolus in the fast inspiration was distributed according to regional airway conductances, these results suggest that in normal subjects differences in airway resistance exist between the upper and lower zones of the upright lung. An early abnormality in bronchitis appears to be a reduction of compliance in the dependent zones, as judged from the decrease in basal ventilation on a slow inspiration.

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