Objective: To assess efficacy of inhaled nitric oxide (INO) on endotoxin induced acute respiratory distress syndrome (ARDS). Methods: Healthy adult dogs (8-11 kg) were anaesthetized, intratracheally intubated and mechanically ventilated to achieve adequate blood gas values. Endotoxin (1 mg/kg) was infused intravenously to induce respiratory failure, and the animals were subsequently treated by either mechanical ventilation (MV, n=6), or MV+INO (n=5). NO flow was provided proximal to the inspiratory limb of the ventilator circuit and regulated by a mass flow controllor. Hemodynamic properties and pulmonary functions were measured. Results: ARDS occured in all the animals within 36 hours after endotoxin infusion as reflected by >60% reduction of PaO2/FiO2, a reduction of dynamic lung compliance (Cdyn) >30%, an increase of Qs/Qt from 5% to 38%, VΔ/VT from 0.15 to 0.57, and pulmonary vascular resistance(PVR) by >200%. Compared to MV, INO at 10, 20, 40 and 80 ppm each for 1 h followed by 10 ppm for 6 h resulted in >50% improvement of PaO2/FiO2, a modest increase of Cdyn, a reduction of Qs/Qt to <25%, VΔ/VT to 0.36, and PVR by 50%, with optimal effects at 20-40 ppm of NO. Radiological and pathological findings confirm therapeutic effects of INO on lung injury. INO didn't have substantial effects on system vascular resistance and cardiac output, and met- hemoglobin was <3%. Conclusion: INO improved gas exchange and PVR in adult dogs with ARDS. (Supported by CMB and Shanghai Education Foundation)