We studied surfactant kinetics on Day 1 of life in 11 preterm infants on mechanical ventilation by infusing stable isotope labeled palmitic (PA) and linoleic acid (LLA). Six infants received exogenous surfactant for the treatment of respiratory distress syndrome (RDS) and five did not meet treatment criteria because of minimal or no disease. The isotopic enrichment of plasma free PA and LLA and of surfactant phosphatidylcholine PA (PC-PA) and LLA (PC-LLA) from tracheal aspirates was measured by mass spectrometry. Significant isotopic enrichment could be measured in PC-PA and PC-LLA from all patients. The fractional synthesis rate (FSR) of PC-LLA was higher than that of PC-PA (22.7 +/- 15.9 versus 12.1 +/- 7.7% per day, p = 0.018). Half-life (HL) of PC-PA was longer than that of PC-LLA (94.7 +/- 18.8 versus 46.6 +/- 32.6 h, p = 0.028). Patients who received exogenous surfactant had longer secretion times (ST) and delayed peak times (PK) but FSR and HL were unaffected. We concluded that: (1) surfactant kinetics can be measured in preterm infants with stable isotope labeled lipids; (2) surfactant FSR and HL calculated with PA and LLA gave different results; (3) patients treated with exogenous surfactant had similar FSRs compared with the nontreated subjects but had longer ST and delayed PK; (4) FSR from plasma free fatty acids (present study) was higher than that from plasma glucose in our previous work (Bunt JEH, Zimmermann LJI, Wattimena D, van Beek R, Sauer PJJ, Carnielli VP. Am J Respir Crit Care Med 1998;157:810-814) in a comparable population of preterm infants with RDS.