Antenatal exposure to intra-amniotic (IA) endotoxin initiates a complex series of events including an inflammatory cascade, increased surfactant production and alterations to lung structure. Using the low frequency forced oscillation technique as a sensitive tool for measurement of respiratory impedance, we aimed to determine which factors contributed most to measured changes in lung mechanics. Respiratory impedance data obtained from sedated preterm lambs exposed to either IA injection with saline or 20 mg endotoxin 1day, 2 days, 4 days, and 15 days prior to delivery at 125 days gestation were studied and association with indices of standard lung morphometry, inflammatory response and alveolar surfactant saturated phosphatidylcholine (Sat PC) pool size were demonstrated. Reduction in tissue impedance with increasing interval between exposure and delivery was evident as early as 4 days after IA endotoxin injection, coinciding with resolution of the inflammatory reaction, increased alveolar surfactant pools and contribution of the alveolar ducts to the parenchymal fraction, and a later decrease in the tissue component of the parenchymal fraction. Decreases in tissue damping (resistance) were more marked than decreases in tissue elastance. Log alveolar Sat PC accounted for most of the variability in tissue damping (88.9%) and tissue elastance (73.4%) whilst the tissue fraction contributed 2% and 6.4% respectively. The alveolar Sat PC pool size was the sole factor contributing to change in tissue hysteresivity. No changes were observed in airway resistance. Despite the complex cascade of events initiated by antenatal endotoxin exposure, variability in lung tissue mechanics is associated primarily with changes in the alveolar Sat PC pool and lung morphology.