The formation of disaturated lecithin in slices of developing rabbit lung was studied using 14C-CDP-choline, 3H-S-adenosylmethionine, and doubly labeled lecithin, lysolecithin and phosphatidylethanolamine as precursors. The labeling patterns suggest that some of the unsaturated lecithin becomes deacylated and subsequently reacylated to form disaturated surface-active lecithin. The rate of this pathway (lysolecithin ‘loop’) increases as a function of gestation and may contribute to the functional maturation of the lung.