1. Starting at 50% vital capacity, boluses of 133Xe were inhaled either at very low or maximal flow rates, the inspiration terminating at total lung capacity (TLC). Flow-dependent changes in bolus distribution were examined by measuring regional radioctivity and computing regional time-constants, and also by recording alveolar plateaux during the subsequent vital capacity expiration. Regional residual volumes, as a fraction of regional total lung capacities, were also calculated. Three groups of subjects were studied: young non-smokers, young cigarette smokers, and older non-smokers.

2. All three groups gave similar results in terms of regional time-constants though there was less variation in the results for young non-smokers.

3. Regional residual volumes also gave similar results in all groups although again there was less variation in young non-smokers. In young smokers, residual volumes in some regions depended on the flow rate used in their measurement, which indicated intraregional inhomogeneity of function.

4. In young non-smokers alveolar plateaux after slow and fast bolus inspirations differed little; this was not the case either in older subjects or in smokers. The increased flow-dependence of the alveolar plateau is analogous with the increased frequency-dependence of dynamic compliance and probably indicates obstruction of small peripheral airways.

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