Maximal expiratory flow (MEF) is controlled by airway cross-sectional area, distensibility, and transmural pressure according to the wavespeed law of flow limitation. We want to elucidate the relationshiup of MEF's to central airway mechanics in man by testing the hypothesis that MEF's of healthy subjects at high lung volumes can be predicted using independent measures of airway size and distensibility. Hyatt et al., (1980) have addressed similar questions using excised human lungs. Central airway cross-section and airway distension during breathing will be measured in 40 to 60 healthy young adults by the method of acoustic reflections measured at the mouth (Fredberg et al. (1980), and will be compared with MEF's by multiple regresssion. The acoustic reflection technique has been developed sufficiently for it to be a useful investigative tool for studies of central airway mechanics, although it remains an imperfectly understood technique in its own right. Refinements of this technique and questions to be addressed in this study include analyzing the sources and nature of errors and artifacts, and carrying out supplemental validation studies. This study initiates a critical evaluation of the acoustic reflection technique taken in combination with and in contrast to an established measure of pulmonary function (MEF). We want to ask how this unique information may contribute to assessment of airways structure and function. This investigation should lead to improved interpretation of maximal expiratory flows and may lead to earlier detection of obstructive airways disease. Diagnosis of obstructive airways disease based upon maximal expiratory flow (MEF) data is hindered by the existence of large variability among healthy subjects that cannot be accounted for by height, weight, vital capacity, or total lung capacity. We want to find out why MEF data have large between-subject variability, and whether that variability can be reduced by correcting flows for airway size. In the long term these studies form the foundation for the application of this technique to localization of airway obstructions of any etiology, to the study of airway reactivity, including responses to drugs and challenge, to the study of airway growth, and to investigation of abnormalities in airway distensibility and its relation to disease.