Cystic fibrosis (CF) is a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) that result in reduced Cl- secretion and increased Na+ absorption, airway surface liquid (ASL) dehydration, decreased mucociliary clearance, infection and inflammation leading to lung injury. Cystic fibrosis patients often present bile acids in the lower airways, however the effects of bile acids on ASL and ion transport in CF airways are not known. Secondary bile acids, such as ursodeoxycholic acid (UDCA), have been shown to modulate immune responses (1,2) and epithelial ion transport (3). Here we investigated the effects of UDCA in normal and CF airway epithelial cell models. NuLi-1 (normal genotype) and CuFi-1 (CF genotype, Δ508/Δ508) primary immortalized airway epithelial cells were grown under an air-liquid interface. Electrogenic transepithelial ion transport was measured as short-circuit current (Isc) across cell monolayers mounted in Ussing chambers. Data are presented as mean ± SEM and were statistically analysed by ANOVA or students t-tests as appropriate. We observed that UDCA (500 mM, 60 min, bilateral) decreased the basal short-circuit current (Isc) in both NuLi-1 and CuFi-1 cells. In NuLi-1 cells basal Isc was reduced by 14 ± 2 mA/cm2 compared to 2 ± 1 mA/cm2 in water-treated control cells (n = 4, p < 0.05). Similarly in CuFi-1 cells, UDCA decreased basal Isc by 7 ± 1 mA/cm2 compared to 0 ± 2 mA/cm2 in water-treated control cells (n = 6, p < 0.05). Interestingly, UDCA significantly decreased the amiloride sensitive current in NuLi-1 cells to 57 ± 10 % of control (n = 4, p < 0.05), suggesting that UDCA inhibited the activity of the epithelial sodium channel (ENaC). Similar effects of UDCA were observed in CuFi-1 cells in which UDCA treatment reduced the amiloride-sensitive currents to 65 ± 4 % of control (n = 6, p < 0.05). The effects of UDCA on amiloride sensitive currents were observed at concentrations as low as 100 mM (n = 4, p < 0.05). In conclusion, we demonstrate a previously unknown effect of UDCA to inhibit ENaC activity in normal and CF human airway epithelial cells suggesting therapeutic potential in CF lung disease.