Abstract： Objective:To investigate the potential role of apoptosis in thedevelopment of acute lung injury (ALI).Methods:Fortyeight Wistar rats were randomly divided into two groups.In ALI group,30 animals were intravenously infused with standard E.colito reproduce the ALI model,and the samples were collected at 4,8 and 12 hours after bacterialc hallenge respectively.In sham operation group,18 animals were intravenously infu s ed with normal saline in equal volume and the samples were obtained at same inte rvals.The parameters of ALI including blood gas,lung index(LI),lung permeability index (LPI),tumor necrosis factorα (TNFα) and pulmonary pathology were determined.Apoptosis was detected by transmission electronmicrosco py,flow cytometry and TdTmediated dUTP nick end labeling (TUNEL),res pectively.Results:It was observed that alveolar spaces were narrowed and neutrophils exuded at 4 h ours after E.coli challenge.With prolongation of time the pulmonary damage was worsened.PaO2 decreased,LI,LPI and TNFα increased,and the differenc es were significant between ALI group and sham operation group (all P<0.01) .TUNEL results showed that in ALI group,a part of cells were positive,and apoptotic cel ls were (7.15±1.26)%,(12.39±1.67)%,(6.72±1.11)% at 4,8 and 12 hours,res pectively,as detected by flow cytometry (all P<0.01).Conclusions:Apoptosis may play an important role in the pathogenesis of ALI.Apoptosis,as a p athogenetic mechanism,accelerates the inflammatory responses of ALI modulated by TNFα.With prolongation of ALI,apoptosis initially increased and th en declined,keeping a low level.