The major molecular species of pulmonary phosphatidylcholine in the fetal rabbit were analyzed as the diacylglycerol acetate derivatives. After fractionation by Ag+ thin-layer chromatography according to the degree of unsaturation, the intact diacylglycerol acetates were analyzed by gas-liquid chromatography to obtain the carbon number composition. The methyl esters of these acetates were used to obtain the fatty acid profiles. The composition of the molecular species was derived from these sets of data. The proportions of 16 : 0/16 : 0, 14: 0/16 : 0 and 16 : 0/16 : 1 increased with gestation, while 16 : 0/18 : 1 decreased. The concentration of 16 : 0/16 : 0 increased about 50% the last two days of gestation while 16 : 0/16 : 1 increased about 300%. The possibility that 16 : 0/16 : 1 is a precursor of 16 : 0/16 : 0 via biohydrogenation is discussed.