Bile acids are often present in the lower airways of people with cystic fibrosis, probably resulting from the aspiration of gastroesophageal refluxate, but the effects of bile acids on airway epithelium have not yet been investigated (1,2). In the colon, bile acids have been reported to acutely stimulate or chronically inhibit epithelial Cl – secretion (3). We investigated the effects of the conjugated secondary bile acid taurodeoxycholic acid (TDCA, 25 μM) on basal transport and carbachol (100 μM) or forskolin (10 μM)-induced ion transport in Calu-3 airway epithelial cells grown in an air-liquid interface and mounted in Ussing chambers. Electrogenic transpeithelial ion transport was measured as short-circuit current (Isc). Data given as Mean ± S.E.M. Statistics were generated using the Student’s paired t-test or one-way Anova analysis, where p ≤ 0.05 is considered significant. We found that acute (5 min) basolateral TDCA treatment of Calu-3 cells stimulated basal Isc by 39 ± 7% (p = 0.0001, n = 13) but had no effect on the Isc responses induced by carbachol or forskolin treatment. The Isc responses to TDCA were abolished in Cl – -free Krebs solution (n = 4) indicating that TDCA modulates Cl – secretion in Calu-3. Prolonged (24hr) TDCA treatment of Calu-3 had no effect on Isc responses to carbachol or forskolin. In Ussing chambers where CFTR Isc was measured, we found that acute treatment with TDCA produced a 32 ± 9% increase in Isc that was abolished by pre-treatment with CFTRinh172 (p = 0.043, n = 5). We also found that TDCA increases Cl – secretion through calcium-activated chloride (CaCC) channels in the apical membrane by 18 ± 7 % (p = 0.01, n = 4). In addition, when Na+/K+ATPase generated currents were measured, acute treatment with TDCA increased Na+/K+pump activity by 13 ± 3%, while pre-treatment with ouabain eliminated this effect (p = 0.0005, n = 9). This further supports TDCA stimulation of secretion in Calu-3 cells. Calcium imaging of Calu-3 cells, grown on glass, revealed that acute treatment with TDCA resulted in a rapid 88 ± 12% increase in calcium mobilization (p = 0.0002, n = 8). The Ca2+ mobilization response to TDCA was abolished in Ca2+ -free buffer indicating that TDCA induced Ca2+ influx into the cell. This study shows for the first time that TDCA modulates Cl – secretion in Calu-3 cells, a model of airway epithelium. Effects of TDCA are dependant upon both the duration and sidedness of exposure to the bile acid. TDCA stimulates Cl – secretion in Calu-3 cells through CFTR and apical CaCC channels. The activation of basolateral Na+/K+ ATPase can indirectly sustain this secretory response. Activation of Cl – secretion in Calu-3 cells could possibly be initiated through intracellular Ca2+ mobilization. This study provides leads for further investigation of the signalling pathways involved in TDCA modulation of airway Cl – secretion.