Objective To find an ideal animal model of acute respiratory distress syndrome (ARDS) through investigating the characteristics of three two-hit animal models of ARDS.Methods Forty-eight SD rats were randomly divided into 4 groups: Control group [2.5 mL/kg normal saline (NS) i.v.given at 0 min and 30 min];OA+OA group [0.5 mL/kg oleic acid (OA) i.v.given at 0 min and 30 min];LPS+LPS group [2.5 mg/kg lipopolysaccharide (LPS) i.v.given at 0 min and 30 min];and OA+LPS group [0.5 mL/kg OA i.v.given at 0 min and 2.5 mg/kg LPS,i.v.given at 30 min].The samples were collected at 5 h after the second drug injection.White blood cells count (WBC),polymorphonuclear leukocyte ratio (PMN%),total protein concentration,tumor necrosis factor α (TNF-α) level in bronchoalveolar lavage fluid (BALF),arterial blood gas analysis and lung wet-dry weight ratio (W/D) were measured,respectively.Pathological changes in the lung tissues were observed and histological scores were evaluated.Results Compared with those in the control group,PaCO2,WBC,PMN%,total protein concentration and TNF-α levels in BALF were significantly increased,while PaO2 was dramatically decreased (P<0.01) in the OA+OA,LPS+LPS and OA+LPS groups.The levels of protein concentration in BALF and lung W/D ratio in the OA+LPS group were significantly higher than these in the LPS+LPS group (P<0.05 for all),but had no statistically significant difference compared with these in the OA+OA group.The levels of WBC,PMN% and TNF-α in BALF in the OA+LPS group were significantly higher than those in the OA+OA group (P<0.05),but not significantly different from those in the LPS+LPS group.The most typical pathological changes and the highest pathological scores were found in the OA+LPS group.Conclusions All the three different methods including OA+OA,LPS+LPS,and OA+LPS can be used to establish “two-hit” animal models of acute respiratory distress syndrome.The “two-hit” animal model of acute respiratory distress syndrome induced by OA+LPS is more closer to clinical ARDS and is useful for studies on the pathophysiology of ARDS,and is an ideal “two-hit” animal model of ARDS.