Palmitic acid is a minor component of natural surfactant and has been used to modify lipid extracts of natural surfactants to optimize theirin vitrosurface properties. The metabolic fate of palmitic acid in surfactant is unknown. The clearance of surfactant-associated radiolabeled palmitic acid after intratracheal administration was investigated with trace doses of surfactant in the adult rabbit and with trace and treatment doses in the 28-d fetal rabbit and the 132-d fetal sheep. Palmitic acid was cleared rapidly from the airways, with less than 2% of the radiolabel recovered as free palmitic acid in the alveolar wash by l h in all models. Recovery as free palmitic acid in the total lung at 2 h was 2% in the adult rabbit and 3% both doses in the preterm rabbit. In the preterm sheep, the recovery as free palmitic acid in the total lung was approximately 2% of the trace dose and 1 % of the treatment dose by 5 h. Between 5 and 15% of the instilled palmitic acid was used as substrate for phospholipid synthesis by the lung in the different models. About 30% of the palmitate derived label was recovered in lipid extracts of liver 30 min after trachéal instillation of labeled surfactant in adult rabbits, whereas only 5—10% of the palmitate derived label was found in liver lipids in the preterm animals. In contrast to palmitic acid, radiolabeled triglycéride was cleared much more slowly from the airspaces and lungs of preterm sheep. Inasmuch as large amounts of palmitic acid are cleared rapidly from airspaces and lung tissue, it will not have a prolonged effect on the surface properties of surfactant but it may serve as a precursor for lung lipid metabolism.