Abstract Background & Aims : Ursodeoxycholate (UDC) stimulates a bicarbonate-rich choleresis, but the cellular mechanisms involved are not fully established. Because ductular secretion also increases biliary HCO 3 − concentration, the purpose of this study was to evaluate whether UDC has direct effects on duct cells by measuring intracellular calcium concentration ([Ca 2+] i) and membrane CI − permeability in Mz-ChA-1 human cholangiocarcinoma cells. Methods : Intracellular calcium levels were measured using fura-2 fluorescence. Membrane CI − permeability was assessed in subconfluent monolayers using 125I efflux and in individual cells using whole-cell patch clamp techniques. Results : Exposure to UDC (2.5 mmol/L) increased [Ca 2+] i from 180 ± 25 to 639 ± 84 nmol/L due to release of Ca 2+ from intracellular stores and stimulated 125I efflux approximately threefold above basal levels. Exposure to extracellular (1.25 mmol/L) or intracellular (100 μmol/L) UDC activated currents carried by CI − ions; intracellular UDC increased current density from 4.7 ± 1.3 to 32.5 ± 8.8 pA/pF. UDC-stimulated currents were inhibited by chelation of intracellular calcium. Conclusions : UDC in pharmacological concentrations increases [Ca 2+] i and stimulates CI − efflux through opening of CI − channels in biliary cells. We speculate that UDC could increase bile flow by direct stimulation of ductular secretion and may be of therapeutic benefit to patients with cystic fibrosis who have impaired adenosine 3′,5′-cyclic monophosphate-dependent biliary secretion.