The phospholipid (PL) transfer proteins are cytosolic proteins that have been characterized by their ability to facilitate the transfer of PL's between membranes in vitro. In mammalian tissues including lung, 3 transfer proteins with different PL specificities have been identified: 1) phosphatidylcholine transfer protein (PC-TP) specific for PC; 2) phosphatidylinositol transfer protein (PI-TP), specific for PI and PC and to lesser extent PG, and 3) nonspecific lipid transfer protein (nsL-TP) which transfers all diacylPLs and cholesterol. In lung, a PL-TP specific for PG has been identified. Because recent evidence suggests that these proteins are involved in regulating phospholipid synthesis and membrane lipid composition, we determined by a vesicle-rat lung mitochondria transfer assay the transfer activities in cytosol (cyt) fractions from adult whole lung and TII from fetal, newborn, and adult rat lung freshly isolated by Nycodenz gradient centrifugation. Transfer activities for all PLs were enriched in adult TII cyt compared with whole lung (12.5-fold (PG), 9.2-fold (PI), 6.5-fold (PC) and 6.6-fold (PE)) (p<0.05 in each case). This suggests that the transfer proteins may play a role in the unique function of the TII cell. All PLs were transferred by d21 fetal>newborn>adult TII. In cyts from cells from different stages, the transfer rate of PG>PI>PC>PE. The relative changes in activities of the PL-TPs during lung development may affect membrane lipid composition and hence, membrane functions. We are currently investigating the role of the PL-TPs in TII PL synthesis. Funded by NIH R29 HL44853.