The aim of the present study was to examine whether neutrophil and its elastase activity played consequential roles in the progression of gut barrier dysfunction during acute alveolar hypoxia by using a specific neutrophil elastase inhibitor, sivelestat. With our institutional approval, 20 male rabbits (weight, 2.0-2.5 kg) were randomly allocated into two groups: control (n = 11) or sivelestat group (n = 9; bolus, 10 mg/kg, followed by 10 mg/kg per hour). At 4 h of alveolar hypoxia exposure (fraction of inspired oxygen, 0.10) under mechanical ventilation, the white blood cell counts and their function to produce oxygen radicals were measured. Intestinal permeability and myeloperoxidase activity were also assessed concurrently with the examination of histological changes of gut mucosa. The examination of sham animals (n = 4) exposed to normoxia was performed under the same study protocol. The circulating leukocyte counts and the neutrophil chemiluminescence were not different between the groups, whereas the neutrophil elastase activity was significantly increased in the control but not in the sivelestat and sham groups. Permeability, leukocyte accumulation, and myeloperoxidase activity of ileal wall in the control group were significantly elevated, accompanied by apparent destruction of gut mucosa compared with the sivelestat group (P < 0.05). Despite no significant differences in systemic inflammatory responses, the neutrophil elastase activity is a key element in the progression of functional and structural injury of gut mucosa during acute alveolar hypoxia.