Nitric oxide (NO) has been shown to play a major role in acute lung injury (ALI) after smoke inhalation. In the present study, we developed an ovine sepsis model, created by exposing sheep to smoke inhalation followed by instillation of bacteria into the airway, that mimics human sepsis and pneumonia. We hypothesized that the inhibition of neuronal nitric oxide synthase (nNOS) might be beneficial in treating ALI associated with this model. Female sheep (n = 26) were surgically prepared for the study and given a tracheostomy. This was followed by insufflation of 48 breaths of cotton smoke (< 40°C) into the airway of each animal and subsequent instillation of live Pseudomonas aeruginosa (5 X 10¹¹ colony forming units [CFU]) into each sheep's lung. All sheep were mechanically ventilated using 100% O₂. Continuous infusion of 7-Nitroindazole (7-NI), an nNOS inhibitor, L-N(G)-Monomethyl-L-arginine (L-NMMA), a non-specific NOS inhibitor, or aminoguanidine (AG), an iNOS inhibitor, was started 1 h after insult. The administration of 7-NI improved pulmonary gas exchange (PaO₂/FiO₂) and pulmonary shunt fraction and attenuated the increase in lung wet-to-dry weight ratio seen in the non-treated sheep. Histologically, 7-NI prevented airway obstruction. The increase in airway blood flow after injury in the non-treated group was significantly inhibited by 7-NI. The increase in plasma concentration of nitrate and nitrite (NOx) was inhibited by 7-NI as well. Post-treatment with L-NMMA improved the pulmonary gas exchange, but AG did not. The results of present study show that nNOS may be involved in the pathogenesis of ALI after smoke inhalation injury followed by bacterial instillation in the airway.