Abstract: Objective To study the effect of mechanical ventilation by breathing machine on the expression of MUC5AC and investigation for mechanism. Methods Rabbits were divide into control group and one, three, six, twelve, twenty-four hours mechanical ventilation groups, and 2, 5, 10cmH2O PEEP mechanical ventilation groups. Contents of TNF-α, IL-8, MUC5AC mRNA and protein in bronchoalveolar lavage fluid were determined by RT-PCR and ELISA methods，respectively. Neutrophil in bronchoalveolar lavage fluid were counted. Neutrophil elastase activity was measured by assays of substrate. Results Mechanical ventilation could increase the expression of MUC5AC in bronchiorum endothelial cells（P<0.05）. After mechanical ventilation for three hour, the expression of TNF-α reached the peak （P<0.05）. The expression of IL-8 reached the peak after six hours（P<0.05）. Neutrophil count and neutrophil elastase activity reached the peak after twelve hours（P<0.05）. With the increase of PEEP, the mRNA and protein of TNF-α, IL-8 ,MUC5AC, the number of PMN and neutrophil elastase activity also would increase（P<0.05）. Conclusion Mechanical ventilation could increase the expression of MUC5AC in bronchiorum endothelial cells. This maybe is concerned with inflammation.