1. Progressive changes were noted in the pool sizes of choline in fetal rabbit lung between 25 and 30 days gestation (term, 31 days) and between 30 days gestation and adult lung. The level of choline in adult lung was double the level in the fetal lung at 25 days gestation. The pool size of choline phosphate decreased 10-fold during this period while the level of CDPcholine decreased by 30%. The phosphatidylcholine content increased 3-fold during development. The major change in the relative pool sizes was a marked decrease in the ratio of choline phosphate to CDPcholine from 26:1 at 25 days gestation to 3.4:1 in adult lung. 2. No differences were detected between the uptake of [14C]choline into slices from fetal lungs at 25 days gestation or slices from adult lung. However, the ability of the adult slices to convert [14C]choline into its derivatives was 30% lower than slices from fetal lung. In addition, whereas fetal slices contained significantly more radioactivity in choline phosphate and CDPcholine, adult slices incorporated significantly more [14C]choline into phosphatidylcholine. Experiments with [3H]choline and 32Pi revealed that the 3H/32P ratio of choline phosphate in fetal or adult slices was identical to the isotopic ratio in phosphatidylcholine, indicating that under the experimental conditions, negligible radioactivity was incorporated by base-exchange. Because of the marked decrease in the pool size of choline phosphate during development, it cannot be concluded that the increase in the incorporation of radioactive choline into phosphatidylcholine is indicative of increased production of phosphatidylcholine by the de novo pathway. The results suggest that if the de novo pathway is responsible for the increase in phosphatidylcholine content, this increase is due to a change in the parameter controlling the flux through the choline phosphate cytidylyltransferase step. The results also indicate that the metabolic flux through choline phosphotransferase is also enhanced during pulmonary development.