Previous studies by us have shown that an increase in the lung surface-active phospholipids is closely related to fetal maturation. The present experiments were performed to determine the relation between fetal maturation and the rate of incorporation of several labelled precursors into phospholipids by lung slices from fetal and newborn lambs.Lung and liver specimens were removed from seven fetal lambs of various gestational ages, nine newborn lambs ranging in age from 1 day to 6 months and from five of the mothers and one non-pregnant ewe. About 500 mg of tissue, mixed with a radioactive precursor in Krebs Ringer Bicarbonate Buffer, was incubated at 38° for three hours. The incorporation of 14C from the various tagged precursors was determined by means of a radiochromatogram scanner after thin layer chromato-graphy.Results showed an increase in the rate of incorporation of14C labelled choline, palmitate and glucose into lung phospholipids, especially lecithin, as the fetus matured. Palmitate incorporation into lecithin was studied; after birth, it was found that the liver was less active than the lung until one month of age, but exceeded the lung in this function by three months after birth. These findings apparently reflect a difference in the development of the enzyme systems involved in phospholipid metabolism in both organs during maturation. From these studies, it is apparent that the lung of the maturing fetus is uniquely prepared for the synthesis of phospholipid, particularly of lecithin. The major significance of the higher rate of biosynthesis of lung lecithin in the fetus near term and newborn may be closely related to active synthesis of surfactant in the lung.