Reactive oxygen species (ROS) can participate in cellular signaling and have been shown to modulate activation of the transcriptional regulatory factor NF-κB. However, the effects of ROS can differ in various cell populations. To examine the role of superoxide in neutrophil activation, we exposed resting neutrophils and neutrophils stimulated with LPS to paraquat, an agent that specifically increases intracellular superoxide concentrations. Culture of resting neutrophils with paraquat resulted in increased production of the proinflammatory cytokines TNF-α and MIP-2, enhanced degradation of IκB-α, and increased nuclear accumulation of NF-κB. Such effects of paraquat were due to intracellular superoxide (O 2 −) since they were blocked by the non-specific antioxidant N-acetyl cysteine and the cell permeable superoxide scavenger Tiron, but not by catalase, which facilitates the conversion of H 2O 2 to H 2O and O 2. Similar potentiating effects of paraquat were found in LPS-stimulated neutrophils. Exposure of neutrophils to paraquat also enhanced phosphorylation of Ser536 in the p65 subunit of NF-κB an event associated with increased transcriptional activity. Examination of kinases critical for LPS-stimulated gene expression showed that addition of paraquat to resting or LPS exposed neutrophils enhanced activation of p38 MAPK, but not that of Akt or ERK1/2. The potentiation of NF-κB translocation and proinflammatory cytokine production, but not of Ser536 p65 phosphorylation, by paraquat was dependent on activation of p38 MAPK. These results demonstrate that increased intracellular superoxide concentrations are proinflammatory in neutrophils, acting through a p38 MAPK dependent mechanism that results in enhanced nuclear accumulation of NF-κB and increased expression of NF-κB dependent proinflammatory cytokines.