Summary: The de novo cytidine-5‘-diphosphocholine (CDP-cholinc) pathway enzymes: cholinc kinase (CK); phosphorylclioline cytidyltransferase (CyT), and phosphorylcholine glyceride transferas (PCGT), and the phospliatidylcholine-lysophosphatidylcholine (PC-lysoPC) cycle pathway enzymes: lysophospholipase (LPL), Ivsophosphatidylcholine-lysophosphatidylcholine acyltransferase (LAT), and acyl-CoA lysophosphatidylcholine acyltransferase (acyl-CoA LAT) were studied in the rabbit lung subcellular fractions. The purity of the fractions was examined by the marker enzymes and electron microscopy. The lamellar bodies had the highest concentration of phospholipids (10.0 μmol/mg protein, 80% of which was phosphatidylcholinc (PC), about 10-fold higher than that of mitochondria (0.8) and microsomes (1.0) (50% of which was PC in both fractions). The lamellar bodies contained no enzymic activities of either the CDP-choline pathway or the PC-lysoPC cycle pathway. The enzymic activities of CK, CyT, LI'L, and LAT were found mainly in the soluble fraction (about 40% for CK and CyT, and 70% for LPL and LAT); PCGT and acyl-CoA LAT were microsomal enzymes. Some general properties of PC-lysoPC cycle enzymes were also studied. The activities of LPL, LAT, and acyl-CoA LAT were not stimulated by the divalent metal ion Ca2+. Their activities were inhibited by 103 M diisopropyl phosphorofluoridate (DFP). The role of the PC-lysoPC cycle pathway enzymes in remodeling the lung PC is discussed. Speculation: The mechanisms regulating PC levels and composition in the lung are unknown. A recent idea is that regulation might occur through remodeling of PC via the PC-lysoPC cycle pathway. Isolation and characterization of the enzymes of this pathway will probe the mecahanisms of remodeling of lung PC and control of this process.