Activation of an innate immune response in airway epithelia by the human pathogen Pseudomonas aeruginosa requires bacterial expression of flagellin. Addition of flagellin (10^-7M) to airway epithelial cell monolayers (Calu-3, airway serous cell-like) increased Cl^- secretion beginning after 3-10 mins, and reaching a plateau after 20-45 mins at I_Cl = 15-50 µA/cm². Similar though 10-fold smaller responses were observed in well-differentiated bronchial epithelial cultures. Flagellin stimulated I_Cl in the presence of maximally stimulating doses of the purinergic agonist ATP, but had no effects following forskolin. Interleukin 1 (IL1, produced by both epithelia and neutrophils during infections) stimulated I_Cl similar to flagellin. Flagellin-, IL1-, ATP- and forskolin-stimulated I_Cl were inhibited by cystic fibrosis transmembrane conductance regulator (CFTR) blockers GlyH101, CFTRinh172 and glibenclamide. Neither flagellin nor IL1 altered transepithelial fluxes of membrane-impermeant dextran (10 kDa) or lucifer yellow (MW = 457), but both activated p38, NF-B and IL8 secretion. Blockers of p38 (SB202190 and SB203580) reduced flagellin- and IL1-stimulated I_Cl by 33-50%, but had smaller effects on IL8 and NF-B. It is concluded that: (i) flagellin and IL1 activated p38, NF-B, IL8 and CFTR-dependent anion secretion without altering tight junction permeability; (ii) p38 played a role in regulating I_Cl and IL8 but not NF-B; (iii) p38 was more important in flagellin- than IL1-stimulated responses. During P. aeruginosa infections, flagellin and IL1 are expected to increase CFTR-dependent ion and fluid flow into and bacterial clearance from the airways. In cystic fibrosis the secretory response would be absent, but activation of p38, NF-B and IL8 would persist.