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The elastic properties of the lung are of primary importance to normal function. A definitive study of these properties entails the simultaneous measurement of pressure and volume under static conditions. A plot of the relationship between pressure and volume in the lungs results in a curve which is approximately linear in the middle part of the vital capacity. The retractive force in this limited part of the curve is a direct function of volume. Although elastic performance is a complex phenomenon, the balance of lung elastic behavior occurs without a delay in time and the majority of the elastic work done on the lung during inspiration can be recovered on expiration. Maj or contributions are made to the over-all elastic behavior of the lungs by surface phenomena and tissue factors. Factors of minor importance include the bronchial and bronchiolar smooth muscle and the amount of blood in the pulmonary vascular bed.For many years the elastic properties of the lungs were attributed to elastic tissue fibers. Surface phenomena were not considered important until 1929 when von Neergaard (1) pointed out that the forces arising from the air liquid interfaces throughout the lung parenchyma could be eliminated by the complete replacement of the gas in the lungs with liquid. He demonstrated that pressure was less in the liquid-filled lung than in the air-filled lung at comparable volumes. He assumed an identical operation of tissue elastic factors in air-and liquid-filled lungs.Radford (2) The purpose of the experiments to be presented was to elucidate further the factors involved in the elastic properties of the lung. The filling of the lungs with mercury metal was studied. It was possible to define the change in surface area of the lungs with accuracy because of the high surface tension of mercury. The equation utilized by Radford (2) was applied in this study:where P is pressure, V is volume, y is surface tension and A is area. The right-hand side of the equation was derived from the Helmholtz equation for the free energy of the surface. It was assumed that temperature was constant and that the quantity and composition of the associated bulk phases were independent of area.Another approach to the problem was to eliminate the tissue elastic properties of the lungs. It was found that after sodium hydroxide extraction and treatment with elastase, the lung no longer exhibited elastic behavior when filled with saline. The pressure-volume measurements with air permitted a study of the geometry of these lung preparations. MATERIAL AND METHODSThe experiments with mercury filling were carried out as follows. Adult mongrel dogs were given pentobarbital anesthesia (25 to 30 mg per kg of body weight). A Neophor valve was connected to a cuffed endotrachga 1515