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Objective: We examined whether mechanical ventilation with low tidal volume induces polymorphonuclear infiltration and proinflammatory and profibrogenic responses in rat lungs compared dependent and nondependent lung region to expression of interleukin-1beta (IL-1beta) and alpha-1 procollagen III (PC III) mRNA., Design: An experimental, randomized and controlled protocol with previously normal rats., Interventions: Three groups of ten animals were studied. Two groups were ventilated (FIO2=0.3) in supine position for 1 h without positive end expiratory pressure, one group with a low tidal volume (6 ml/kg), and the other with a high tidal volume (24 ml/kg). In the third group animals were kept in spontaneous ventilation for 1 h., Measurements and Results: After ventilation the right lung was used to quantify polymorphonuclear infiltration. The left lung was divided into dependent and nondependent regions, and expression of IL-1beta and PC III mRNA was quantified by northern blot analysis. The group ventilated with low tidal volume had greater polymorphonuclear infiltration IL-1beta and PC III mRNA expression than the nonventilated group. Similar results were observed with high tidal volumes. There was no difference between low and high tidal volume ventilation. Expression levels of IL-1beta and PC III mRNA were higher in the nondependent region of ventilated groups and equal in the nonventilated group., Conclusions: Even a low tidal volume mode of mechanical ventilation induces proinflammatory and profibrogenic response, with a nondependent predominance for IL-1beta and PC III mRNA expression in supine, ventilated, previously normal rats.