While choline, ethanolamine and methionine are precursors of phospholipids — especially phosphatidylcholine, the major component of lung surfactant — there have been very few studies reporting their levels in the lung. Using an enzymatic assay for choline, we have for the first time determined its level in adult rat lung and in fetal rat lung at 19.5, 21 and 22 days of gestation. The average lung choline content in adult rats was found to be 32.8 nmol/g of lung or 5.1 nmol/mg DNA. There is no difference in lung choline concentration between fetal lungs and lungs of adult rats. Adult lung content of ethanolamine and methionine is similar to that of choline, but fetal lungs appear to have higher levels of ethanolamine (10.0 nmol/mg DNA) and methionine (12.1 nmol/mg DNA) as compared to adult lungs. Postmortem changes in lung choline levels are not as significant as those reported to occur in brain.