Objective To evaluate the role of MLK3-MKK3/6-p38MAPK signal transduction pathway in lipopolysaccharide (LPS)-induced acute lung injury (ALI) in rats. Methods Seventy-eight healthy adult male SD rats weighing 200-250 g were randomly divided into 4 groups: control group (group C, n = 6), ALI group ( n =24), MLK3 inhibitor K252a group (group MK, n = 24) and p38MAPK specific inhibitor SB203580 group (group MS, n = 24). ALI was induced by iv injection of LPS 5 mg/kg via tail vein, while the equal volume of normal saline was given instead in group C. K252a 75 μg/kg and SB203580 10 mg/kg were injected intravenously via tail vein 30 min before LPS administration in group MK and MS respectively. The rats were killed at 1, 3, 6 and 12 h (T1-4) after LPS administration in group ALI, MK and MS (6 rats at each time point) and immediately after normal saline administration in group C. The lungs were removed for microscopic examination. The left lung was lavaged.The broncho-alveolar lavage fluid (BALF) was collected. The concentration of TNF-α in the BALF was determined by ELISA. W/D lung weight ratio was calculated. The expression of p-MLK3, p-MKK3/6 and p-p38MAPK were determined by Western blot. Results The concentration of TNF-α in the BALF, W/D lung weight ratio, and expression of p-MLK3, p-MKK3/6 and p-p38MAPK were significantly higher in the other three groups than in group C (P ＜ 0.01). The parameters mentioned above were significantly lower in group MK, and the concentration of TNF-α in the BALF, W/D lung weight ratio, and p-p38MAPK expression were significantly lower in group MS than in group ALI (P ＜ 0.05). The microscopic examination showed that LPS-induced ALI was less severe in group MK and MS than in group ALI. Conclusion MLK3-MKK3/6-p38MAPK signal transduction pathway plays an important role in LPS-induced ALI in rats.