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Ventilator-induced lung injury (VILI) is a major cause of morbidity and mortality in intensive care units. The stress-inducible gene product, heme oxygenase-1, and carbon monoxide (CO), a major byproduct of heme oxygenase catalysis of heme, have been shown to confer potent antiinflammatory effects in models of tissue and cellular injury. In this study, we observed increased expression of heme oxygenase-1 mRNA and protein in a rat model of VILI. To assess the physiologic function of heme oxygenase-1 induction in VILI, we determined whether low concentration of inhaled CO could serve to protect the lung against VILI. Low concentration of inhaled CO significantly reduced tumor necrosis factor-␣ levels and total cell count in lavage fluid, while simultaneously elevating levels of antiinflammatory interleukin-10 levels. To better characterize the mechanism of CO-mediated antiinflammatory effects, we examined key signaling pathways, which may mediate CO-induced antiinflammatory effects. We demonstrate that inhaled CO exerts antiinflammatory effects in VILI via the p38 mitogen-activated protein kinase pathway but independent of activator protein-1 and nuclear factor-B pathways. Our data lead to a tempting speculation that inhaled CO might be useful in minimizing VILI. Keywords: cytokines; heme oxygenase-1; p38 MAPKAccording to an international study, an average of 39% of intensive care unit patients requires mechanical ventilation worldwide (1). Many of these patients develop ventilator-induced lung injury (VILI) (2). Eventually VILI contributes to acute respiratory distress syndrome (ARDS), which has a 40 to 50% mortality rate (3). Although clinical trials showed that ARDS/VILI-related mortality could be attenuated with lower tidal volume ventilation, positive end-expiratory pressure (PEEP) ventilation, and more recently, with recruitment maneuver combined with protective ventilation strategy, the syndrome remains a major problem in intensive care units (3)(4)(5).It is established that mechanical ventilators that apply high volumes and pressures can lead to increased alveolar-capillary permeability (6). This loss of compartmentalization results in increased fluid influx to the alveoli from the capillaries, causing pulmonary edema. The injured or ruptured cells attract neutrophil leukocytes and activate alveolar macrophages, causing inflammation in the lung (6). Tremblay and colleagues have suggested that ventilation can provoke an inflammatory response in the distal airway and alveolar cells (7), manifested by increased production of proinflammatory cytokines. It is speculated that the released proinflammatory cytokines could enter the circulation causing inflammation in other systemic organs. In addition, the previously diseased or injured lungs are more susceptible to subsequent mechanical ventilation, releasing more proinflammatory cytokines than healthy lungs, perhaps reflecting the cumulative effects of multiple injuries (8, 9). Lipopolysaccharide (LPS), acid aspiration, and cecal ligation/perforation-induced sep...