The lung transfer components of 70 normal, adult nonsmokers 18 to 78 years of age were measured by the single-breath diffusing capacity of the lung for CO. The membrane conductance and the pulmonary capillary blood volume were estimated by several measurements of the diffusing capacity at increasing alveolar PO2. Diffusing capacity, membrane conductance, and pulmonary capillary blood volume are functions of biometric references (height or alveolar volume) and of age. The observed decrease with age was more evident after 40 years of age for diffusing capacity and membrane conductance. Pulmonary capillary blood volume decreases at a later age. The changes in membrane conductance and pulmonary capillary blood volume with age may be due either to morphologic changes in the alveolar capillary surface or increasing inhomogeneities of distribution or both.