The uptake and metabolism of [3H]leucine, [U-14C]glucose, and [3H]palmitate were studied in rabbits aged --3, 1, 7, 28, and 90 days up to 1--1.5 years. Although lung composition did not change markedly, there were great differences in lung metabolism between the perinatal period (late fetal and newborn) and later stages of development. Leucine incorporation into lung protein was highest (3.1--3.4 nmol/mg protein/hr) in late fetal and newborn rabbits and decreased rapidly thereafter. Palmitic acid incorporation decreased during the first week after birth from 150 nmol/100 mg/hr to 85 nmol/100 mg/hr at 7 days of age; it increased thereafter to 170 nmol/100 mg/hr at 4 weeks of age and remained at that level throughout the entire period studied. Glucose uptake and lactate production were higher in fetal lungs than in all other age groups. Lipid biosynthesis from glucose was 2--4 times higher in fetal lungs than at all other ages; furthermore, more than 60% of glucose carbon atoms channeled into lipid was incorporated into fatty acids, whereas at all other ages glucose was chiefly a precursor of lipid glycerol.