SummaryWe investigated the relative contribution of the two pathways of phosphatidylcholine (PC) synthesis in lungs from chick embryos of 12 to 20 days of incubation as well as from hatched chicks and adult chickens. Lung “blocks” were incubated for 60 minin vitroin the presence of [3H]methionine and [14C]choline. PC was separated by thin‐layer chromatography, and the amount of labeled product was determined by liquid scintillation counting. PC synthesis (expressed as pmoles PC per mg protein per 60 min) by the choline pathway declined from the 12‐day value of 40 to a minimum of 8 at day 16 and then rose gradually to a peak of 74 on day 1 posthatching. PC synthesis via the phosphatidylethanolamine pathway also declined from day 12 (54) to day 16 (13) but peaked on day 19 (65) and again on day 1 posthatching (54). Synthetic activities of the two pathways differed significantly (P≤ 0.05) only on days 14, 18, and 19 of incubation. Unlike the mammalian lung, neither pathway seems to predominate throughout incubation or after hatching. The increase in PC synthesis towards the end of incubation correlates with the time of appearance of lamellar bodies in the lung epithelial cells and of surfactant in the airways.SpeculationBecause normal morphologic and biochemical development of the chick lung resembles that occurring in the mammalian lung in many respects, conclusions drawn from the study of avian lungs may well be germane to an appreciation of mammalian lung differentiation in general. Information concerning regulation of the biosynthetic pathways for phosphatidylcholine in the avian embryo may provide some clues for improved clinical treatment of certain pulmonary disorders, including respiratory distress syndrome of the newborn.