The thickness and composition of the walls of nonrespiratory bronchioles were quantitatively assessed in groups of normal and emphysematous lungs. In both the groups there was a gradual increase in the thickness of the walls of nonrespiratory bronchiles with increasing caliber, but this change was not proportionate. Smaller bronchioles possessed thicker walls in both the groups. Bronchioles of comparable internal diameter were also found to be slightly thicker than normal in emphysematous specimens at all levels of caliber. Through a system of schematic models, it was shown that the relatively thicker walls of smaller bronchiles in normal and emphysematous lungs would enhance airflow in the proper direction both during expiration and inspiration. The slightly thicker walls of bronchioles in emphysema would result in their closure at substantially higher lung volumes than would occur with the somewhat thinner walled bronchioles of corresponding diameters in normal lungs. Although such thickening might only be focal in emphysema, this would interfere with expiratory airflow and result in air-trapping. This investigation failed to demonstrate differences in the composition in the walls of nonrespiratory bronchioles of normal and emphysematous lungs.