Earlier studies indicated a role for reactive oxygen species (ROS) in host defense againstPseudomonas aeruginosainfection. However, the role of nicotinamide adenine dinucleotide phosphate-reduced (NADPH) oxidase (NOX) proteins and the mechanism of activation for NADPH oxidase inP. aeruginosainfection are not well-defined. Here, we investigated the role of NOX2 and NOX4 proteins inP. aeruginosainfection, ROS generation, and endothelial barrier function in murine lungs and in human lung microvascular endothelial cells (HLMVECs). Airway instillation ofP. aeruginosastrain 103 (PA103) significantly increased ROS concentrations in bronchial alveolar lavage (BAL) fluid, along with the expression of NOX2 and NOX4, but not NOX1 and NOX3, in lung tissue. In addition, PA103-infected HLMVECs revealed elevated concentrations of ROS, NOX2, and NOX4. In murine lungs and HLMVECs, PA103 induced the NF-κB pathway, and its inhibition blocked PA103-dependent NOX2 and NOX4 expression. Barrier function analysis showed that heat-killed PA103 induced endothelial permeability in a dose-dependent manner, which was attenuated by treatment with small interfering (si)RNA specific for NOX4, but not NOX2. Furthermore, the knockdown of NOX4, but not NOX2, with siRNA reduced PA103-mediated apoptosis in HLMVECs.In vivo,the down-regulation of NOX4 with NOX4 siRNA attenuated PA103-induced lung vascular permeability. The deletion of NOX2 in mice exerted no effect on permeability, but offered significant resistance toP. aeruginosa-induced lung inflammation. These data show thatP. aeruginosalung infection up-regulates NOX2 and NOX4 expression and ROS generation, which play distinct roles in regulating lung inflammation, apoptosis, and permeability.