Although neutrophils are a critical component of the inflammatory process, their functional regulation is incompletely understood. Of note, although pCO2 varies physiologically and pathologically in the neutrophilic milieu, its affect on neutrophil biological processes is unresolved. We demonstrate here that neutrophils respond to hypo- and hypercarbia, (0.04% and 10%) by increasing and decreasing, respectively, intracellular oxidant production (basally and in response to opsonized Escherichia coli and phorbol esters). Further, hypo- and hypercarbia increase and decrease, respectively, the release of IL-8 from LPS-stimulated cells; both effects are attenuated by the carbonic anhydrase inhibitor, acetazolamide. Anion exchange did not restore pH(i) under hypocarbic conditions, however partial restoration of pH(i) under hypercarbic conditions was achieved by Na+/H+ exchange and vacuolar ATPases. Abrogation of pCO2-induced changes in pH(i) prevented hypocarbia-induced generation of reactive oxidant species. These observations suggest that CO2 modifies neutrophil activity significantly by altering pH(i).