The objective of this proposal is primarily concerned in elucidating the biochemical and morphologic pathways of surfactant synthesizing systems in the lungs of adult, fetal and newborn rabbit to gain insights in the understanding of the respiratory distress syndrome of newborn infants and in other conditions known to be associated with pulmonary atelectasis. Emphasis of this proposal is placed in the demonstration of alveolar lining layer consistently in a large number of alveoli as an initial step. It has been shown that the principal surface-active lecithin is heterogeneous; dipalmitoyl and palmito- myristic lecithins. The former is synthesized by triple methylation of phosphatidyl enthanolamine, the latter by CDP-choline + D alpha, beta diglyceride. In the early fetal life of rabbit CDP-choline pathway predominates, while in the late fetal life transmethylation occurs most. In addition, our studies indicate that even in a given individual rabbit there are two distinctive turn over rates of injected palmitate. With these informations, it is feasible to elucidate two synthetic pathways more in details both biochemically and morphologically by utilizing autoradiographic technique on palmitate and myristate. The study will further extend in demonstration the protein transport across the alveolar wall and its contribution to the normal and abnormal alveolar lining layer. By studying the conditions associated with atelectasis by utilizing the same technique, it appears possible now to pinpoint the site(s) of injury at biochemical and morphological levels.