The lungs of intrauterine 135--136-day-old lambs were lavaged with amnionic fluid, with or without meconium, to determine the effect on lung phosphatidylcholine (PC) concentration, synthesis, and function. No differences were apparent between animals lavaged with amnionic fluid or amnionic fluid with meconium. When lavaged lungs were compared to nonlavaged controls, no detectable differences were observed in histology or the quantity of saturated (SPC) and unsaturated phosphatidylcholine (UPC). However, the lavaged lungs retained a larger fraction of maximal lung volume at 5 cm H2O distending pressure and the incorporation of (32P) orthophosphate into lung PC was significantly reduced. In addition, two lavaged animals who became acidotic (pH less than 7.20) exhibited decreased incorporation of 14C) palmitate into whole lung unsaturated, and saturated phosphatidylcholine. CONCLUSIONS These data indicate that amnionic fluid can reduce lamb lung de novo synthesis of PC and may contribute to the alterations in lung PC found in neonatal syndromes of respiratory distress. Acidosis may accentuate this effect.