Abstract： Objective To explore the role of hydrogen sulfide/cystathionine-γ-lyase (H2S/CSE) system in lipopolysaccharide (LPS)- induced acute lung injury (ALI) in rats and the underlying mechanisms. Methods Sixty-four Sprague-Dawley (SD) rats were randomly divided into four groups: control, LPS (instilled intratracheally to induce ALI), sodium hydrosulfide (NaHS), propargylglycin (PPG). Animals were sacrificed at 4 and 8 hours (n = 8) after administration of the above agents. Morphological changes in lung tissues were determined, H2S, nitrogen monoxide (NO) and carbon monoxide (CO) concentration in plasma were determined. Malondialdehyde (MDA) content, and myeloperoxidase (MPO), CSE, inducible nitric oxide synthase (iNOS), hemeoxygenase (HO) activity of the lung were also determined. The level of P-selectin of lung tissue was measured by radioimmunoassay. Immunohistochemisty technique was performed to examine the expression of iNOS and HO-1 protein in lung tissues. Results Severe injuries of lung tissues and raised MDA content, MPO activity and P-selectin level were observed in rats treated with LPS. LPS also led to a drop in plasma H2S concentration and lung CSE activity. The enzyme activity of iNOS and HO, and their protein expression, plasma NO, and CO levels increased after LPS instillation (P＜0. 05 or P＜0. 01). Pre-administration of NaHS before LPS could attenuate the changes induced by LPS. Pre-administration of PPG exacerbated the injuries induced by LPS, with increased MDA content, MPO activity, P-selectin level, the plasma NO level, lung iNOS activity and its protein expression, but there was no prominent variation in CO level, HO activity and HO-1 protein expression compared with those of LPS group. Conclusion Downregulation of H2S/CSE is involved in the pathogenesis of ALI induced by LPS. Endogenous and exogenous H2S provide protection against ALI, which may be explained by its anti-oxidative effects, attenuation of inflammatory over-reaction in lung induced by polymorphonuclear neutrophils, downregulation of NO/iNOS system and the upregulation of CO/HO-1 system.