Abstract： Objective To observe the expression of heat shock protein 70 (HSP70) in lung of rats after paraquat (PQ) poisoning and to investigate the therapeutic effects of ulinastatin. Method Seventy-two adult healthy SD rats were randomly(random number) divided into control group (group A, n = 24), poisoning group (group B, n =24) and ulinastatin group (group C, n =24). The rat models of acute PQ poisoning were established by intra-gastric administration of 80 mg/kg PQ to rats of group B and group C, and the rats of group C were intra-peritoneally injected with 100 000 IU/kg ulinastatin 30 minutes after poisoning. The expression of HSP70 in lung tissue was observed, and W/D and histopathological changes in lung tissue were compared 12 h,24 h,48 h and 72 hours after poisoning. The expression of HSP70 in lung tissue was assayed by using RT-PCR. All quantitative data were processed with one-way analysis of variance to compare multiple sample means. Results Compared with group A, the expression of HSP70 in the lung of rats in group B and group C increased significantly at all intervals ( P ＜ 0.05). The pathological changes in lung tissue of rats with PQ poisoning showed mainly congestion,leukocytes infiltration and local hemorrhage, and the pathological changes in lung tissue of group C were significantly lessened. Conclusions Ulinastatin may ameliorate the acute lung injury to a certain extent after PQ poisoning in rats by enhancing the expression of HSP70.