The purpose of this study was to determine the role of NADPH oxidase in H + secretion by airway epithelia. In whole cell patch clamp recordings primary human tracheal epithelial cells (hTE) and the human serous gland cell line Calu-3 expressed a functionally similar zincblockable plasma membrane H + conductance. However, the rate of H + secretion of confluent epithelial monolayers measured in Ussing chambers was 9-fold larger in hTE compared with Calu-3. In hTE H + secretion was blocked by mucosal ZnCl 2 and the NADPH oxidase blockers acetovanillone and 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), whereas these same blockers had no effect in Calu-3. We determined levels of transcripts for the NADPH oxidase transmembrane isoforms (Nox1 through -5, Duox1 and -2, and p22 phox ) and found Duox1, -2, and p22 phox to be highly expressed in hTE, as well as the intracellular subunits p40 phox , p47 phox , and p67 phox . In contrast, Calu-3 lacked transcripts for Duox1, p40 phox , and p47 phox . Anti-Duox antibody staining resulted in prominent apical staining in hTE but no significant staining in Calu-3. When treated with amiloride to block the Na + /H + exchanger, intracellular pH in hTE acidified at significantly higher rates than in Calu-3, and treatment with AEBSF blocked acidification. These data suggest a role for an apically located Duox-based NADPH oxidase during intracellular H + production and H + secretion, but not in H + conduction.