This study addresses the fundamental anatomy and functon of the tension-bearing structures of the lung. Which structures in the lung bear tensile forces? How are they connected? What is the extent and nature of their mechanical interactions? How do they determine the configuraton of airspaces and the magnitude of lung recoil? Understanding of these basic physiological mechanisms is relevant to the pathophysiology of disease in which alveolar surface tension or the mechanical properties of the interstitial tissues are changed. Lungs will be prepared by intravascular fixation and dehydration for light and electron microscopy. Variables will be lung volume, lung volume history, surface tension, and species. The configuration of the alveolar duct and its associated alveoli will be assessed quantitatively by measuring dihedral angles at junctions of alveolar septa, waviness of fibrous tissues, and pleating of elveolar septa. Certain anatomic features (alveolar entrance rings, septal ridges, and septal junctions) will be identified and their lengths measured stereologically. The distributions of collagen and elastin within the alveolar ductal unit will be quantified. These data will give direct evidence bearing on the conditions for slackness of septal fibrous tissues, the anatomy and functional role of alveolar ductal and septal ridge fibrous tissues, distinctions between the roles of collagen and elastin and functional distinctions between radial and non-radial septa. A more secure, though more complex, picture of the anatomy and mechanics of the lung parenchyma should emerge.