Lung deformability has an important effect on pulmonary function. For example, a nonuniform deformation can impair gas exchange or change pulmonary vascular resistance. In analyzing these deformations, it is necessary to know the mechanical properties of the lung. The objective of this study is to formulate a constitutive equation that describes mechanical properties of the lung. The simplest form of the constitutive equation that characterizes uniform and nonuniform deformations of an elastic solid is the linear equation, known as Hooke's law. In the first part of this project, Hooke's law for lung parenchyma tissue will be derived from measurements of the linear elastic coefficients, the shear modulus and the bulk modulus, of the liquid-filled rabbit lung. Previously developed experimental techniques will be used in these measurements; the punch indentation test for measuring the shear modulus, and superposition of small pressure-volume loops on the main pressure-volume curve to measure the bulk modulus. By comparing the elastic moduli of the liquid-filled lung with those obtained from the air-filled lung, it will be possible to assess the significance of the parenchymal tissue network in resisting volume and shape distortions of the lung, as opposed to the interfacial surface tension. In the second part of the project, the microstructural model of the lung of Wilson and Bachofen will be extended by including previously neglected network of the alveolar walls. A constitutive equation will be derived from the extended model. This equation would yield better quantitative predictions of large and small nonuniform deformations of the lung than the predictions obtained from the model in which the alveolar membranes have been neglected.