In the rabbit gallbladder epithelium, hydrochlorothiazide (HCTZ) was shown to inhibit the transepithelial NaCl transport and the apical Na(+)-Cl- symport, to depolarize the apical membrane potential and to enhance the cell-to-lumen Cl- backflux (radiochemically measured), this increase being SITS-sensitive. To better investigate the causes of the depolarization and the Cl- backflux increase, cells were punctured with conventional microelectrodes on the luminal side (incubation in bicarbonate-free saline at 27 degrees C) and the apical membrane potential (Vm) was studied either with prolonged single impalements or with a set of short multiple impalements. The maximal depolarization was of 3-4 mV and was reached with 2.5 x 10(-4) M HCTZ. It was significantly enhanced by reducing luminal Cl- concentration to 30 mM; it was abolished by SCN-, furosemide, SITS; it was insensitive to DPC. SITS converted the depolarization into a hyperpolarization of about 4 mV; this latter was apamin, nifedipine and verapamil sensitive. It was concluded that HCTZ concomitantly opens apical Cl- and (probably) Ca2+ conductances and, indirectly, a Ca(2+)-sensitive, apamin inhibitable K+ conductance: since the intracellular Cl- activity is maintained above the value predicted at the electrochemical equilibrium, the opening of the apical Cl- conductance depolarizes Vm and enhances Cl- backflux. In the presence of apamin or verapamil, to avoid the hyperpolarizing effects due to HCTZ, the depolarization elicited by this drug was fully developed (7-10 mV) and proved to be Ca2+ insensitive. On this basis and measuring the transepithelial resistance and the apical/basolateral resistance ratio, the Cl- conductance opened by HCTZ has been estimated and the Cl- backflux increase calculate: it proved to be in the order of that observed radiochemically. The importance of this Cl- leak to the lumen in the overall inhibition of the transepithelial NaCl transport by HCTZ has been evaluated.