The role of endogenous sulfur dioxide (SO 2 ), an efficient gasotransmitter maintaining homeostasis, in the development of acute lung injury (ALI) remains unidentified. We aimed to investigate the role of endogenous SO 2 in the pathogenesis of ALI. An oleic acid (OA)-induced ALI rat model was established. Endogenous SO 2 levels, lung injury, oxidative stress markers and apoptosis were examined. OA-induced ALI rats showed a markedly downregulated endogenous SO 2 /aspartate aminotransferase 1 (AAT1)/AAT2 pathway and severe lung injury. Chemical colorimetry assays demonstrated upregulated reactive oxygen species generation and downregulated antioxidant capacity in OA-induced ALI rats. However, SO 2 increased endogenous SO 2 levels, protected against oxidative stress and alleviated ALI. Moreover, compared with OA-treated cells, in human alveolar epithelial cells SO 2 downregulated O 2 À and OH À generation. In contrast, L-aspartic acid-b-hydroxamate (HDX, Sigma-Aldrich Corporation), an inhibitor of endogenous SO 2 generating enzyme, promoted free radical generation, upregulated poly (ADP-ribose) polymerase expression, activated caspase-3, as well as promoted cell apoptosis. Importantly, apoptosis could be inhibited by the free radical scavengers glutathione (GSH) and N-acetyl-L-cysteine (NAC). The results suggest that SO 2 /AAT1/AAT2 pathway might protect against the development of OA-induced ALI by inhibiting oxidative stress.