Pulmonary surfactant is of vital importance to the lungs where it serves to prevent collapse of the alveoli and distal airways at low lung volumes. Pulmonary surfactant is a complex mixture of lipids and proteins which is synthesized in the alveolar Type II cells and stored in the form of cytoplasmic organelles known as lamellar bodies. Secretion of the lamellar bodies into the alveoli results in dispersion of the lamellar bodies and the formation of a film of surface-tension lowering phospholipids over the surface of the alveoli and distal airways of the lungs. Very little is known about the mechanisms through which biosynthesis, storage and secretion of pulmonary surfactant are controlled. Pulmonary surfactant exists in two pools, intracellular and extracellular. By measuring the sizes of these two pools as a function of age, we have sought to establish a relationship between them. Using toxic agents as a means of manipulating pool sizes, we seek to establish the limits of that interpool relationship.