OBJECTIVE: Pulmonary surfactant protein-D(SP-D) is regarded as a valuable biomarker in acute lung injury(ALI) and acute respiratory distress syndrome (ARDS), but the alterations of SP-D in lung tissues in the early course of ALI remain unknown.This study was designed to explore the temporal fluctuations of SP-D and SP-D mRNA in young rats with ALI induced by lipopolysaccharide(LPS),as well as the alterations of ultrastructures of alveolar type Ⅱ(ATⅡ) cells. METHODS: Rat ALI models were established by intraperitoneal injection of LPS (4 mg/kg). The rats were sacrificed at 6,12, 24, 36, 48 and 72 hrs after LPS injection (8 rats each time point). Western blot and RT-PCR were employed to detect the contents of SP-D and SP-D mRNA in lung tissues. The ultrastructures of ATⅡcells were studied with transmission electron microscopy. RESULTS: Both SP-D mRNA and SP-D levels decreased after 12 hrs of LPS administration. The SP-D mRNA level reached a nadir at 24-36 hrs, but the SP-D level was reduced to its nadir by 48 hrs after LPS administration.LPS resulted in the alterations of lamellar bodies (LBs) in size (multilamellar forms), density(vacuole-like deformity) and number. The alterations of ultrastructures of ATⅡcells were most significant at 48 hrs. The clinical symptoms of ALI rats were most severe at 48 hrs. CONCLUSIONS: The alterations of the SP-D level were time-dependent in the early course of LPS-induced ALI. The lowest level of SP-D occurred at 48 hrs while severe multideformities of ATⅡcells were presented. A decreased level of SP-D in the lungs in the early stage of ALI may be associated with a worse clinical outcome.