Chronic lung injury in the neonate is termed bronchopulmonary dysplasia (BPD). These patients generally require supplemental oxygen therapy, and hyperoxia has been implicated in the pathogenesis of BPD. The concomitant use of oxygen and inhaled NO (iNO) may result in the generation of reactive nitrogen species or may have an anti-inflammatory effect in the neonatal lung. We tested the hypothesis that exposure to >95 O2 in neonatal mice would increase trafficking of leukocytes into the lung and that the addition of iNO to >95 O2 would decrease this leukocyte trafficking. Hyperoxia resulted in fewer alveoli, increased presence of neutrophils and macrophages, and decreased number of mast cells within the lung parenchyma. Adding iNO to hyperoxia prevented the hyperoxia-induced changes and resulted in the numbers of alveoli, neutrophils, macrophages, and mast cells approximating those found in controls (room air exposure). Intercellular adhesion molecule (ICAM) and monocyte chemotactic protein-1 (MCP-1), two factors responsible for leukocyte recruitment, were up-regulated by hyperoxic exposure, but the addition of iNO to the hyperoxic exposure prevented the hyperoxia-induced up-regulation of ICAM and MCP-1. These data demonstrate that iNO alters the hyperoxia-induced recruitment of leukocytes into the lung.