The authors have investigated the morphological differences responsible for the variability in two tests of pulmonary function, maximal expiratory flow rates (MEF) and the frequency dependence of dynamic compliance (CDYN ratio). Functional measurements were obtained from 53 normal and minimally diseased postmortem human lungs. Morphological measurements performed on these same lungs included airway diameter at three levels in the bronchial tree, the amount of bronchial gland mass, and the alveolar surface to volume ratio. Multiple regression analysis suggests that the diameter of the peripheral conducting airways (membranous bronchioles) is the major morphological determinant for both MEF and the CDYN ratio in lungs at any particular age. Age dependent changes in both functional tests were associated primarily with differences in the alveolar surface to volume ratio. Minimal emphysema and a lesion associated with cigarette smoking, respiratory bronchiolitis, have no demonstrable effect on either MEF or the CDYN ratio. These studies provide further evidence that the peripheral conducting airways are a major determinant of ventilatory function in the normal human lung.