Objective:To evaluate the role of Toll-like receptor 4 (TLR4) in the mechanism by penehyclidine hydrochloride alleviating lipopolysaccharide (LPS)-induced acute lung injury(ALI)in rats.Methods:Twenty-four healthy male Sprague-Dawley rats, aged 10 weeks, weighing 220-250 g, were divided into 4 groups ( n=6 each) according to the random number table method: normal saline group (NS group), endotoxin-induced ALI group (ALI group), penehyclidine hydrochloride + normal saline group (PHC+ NS group) and penehyclidine hydrochloride + endotoxin-induced ALI group (PHC+ ALI group). ALI was induced by intratracheal instillation of LPS 5 mg/kg in anesthetized animals. In group PHC+ ALI, penehyclidine hydrochloride 2 mg/kg was intraperitoneally injected immediately after intratracheal instillation of LPS. The equal volume of normal saline was injected into the airway in group NS, and penehyclidine hydrochloride 2 mg/kg was intraperitoneally injected immediately after intratracheal instillation of normal saline in group PHC+ NS. The rats were sacrificed at 6 h after intratracheal instillation of LPS or normal saline, and lungs were removed. The lung was lavaged and broncho-alveolar lavage fluid (BALF) was collected for determination of concentrations of tumor necrosis factor-alpha (TNF-α) and interleukin-1beta (IL-1β) by enzyme-linked immunosorbent assay. Lung tissues were obtained for microscopic examination of the pathological changes (with a light microscope) and for determination of wet to dry lung weight ratio (W/D ratio) and TLR4 protein and mRNA expression (by immuno-histochemistry or real-time polymerase chain reaction). Results:Compared with group NS and group PHC+ NS, the W/D ratio and concentrations of TNF-α and IL-1β in BALF were significantly increased, the expression of TLR4 protein and mRNA in lung tissues was up-regulated ( P<0.01), and the pathological changes of lung tissues were aggravated in group ALI . Compared with group ALI, the W/D ratio and concentrations of TNF-α and IL-1β in BALF were significantly decreased, the expression of TLR4 protein and mRNA in lung tissues was down-regulated ( P<0.01), and the pathological changes of lung tissues were significantly mitigated in group PHC+ ALI. Conclusions:The mechanism by which penehyclidine hydrochloride reduces endotoxin-induced lung injury may be related to reduction of TLR4 activity and thus inhibition of pulmonary inflammatory responses in rats.