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1. When bathed with Ringer solutions on both sides in Ussing chambers, isolated urinary bladders of trout carry out an electrically silent transport of Na+ and Cl- ions from mucosa to serosa. In the present experiments, an electrical gradient was set up across the epithelium mounted under these conditions either by using a voltage-clamp technique or by submitting the mucosal side to the polyene antibiotic Amphotericin B. 2. With stable potential differences between -40 and +40 mV applied to the tissue, both unidirectional fluxes (Jms and Jsm) for Na+ and for Cl- varied as a function of the electrical gradient. This indicates that a sizeable fraction of these ionic exchanges is due to passive diffusion across a low-resistance pathway. 3. Amphotericin B (40 micrograms/ml.) applied to the mucosal side induced an immediate and large (up to 50 mV, serosa positive) potential difference and a sustained diminution in tissue resistance. These effects were strictly Na+-dependent and were reduced in the presence of ouabain. Therefore they are due to increased passive entry of Na+ along its electrochemical gradient across the cellular apical membrane. 4. The effect of Amphotericin B was enhanced when Cl- was replaced by an impermeant anion such as gluconate. Therefore the shunting effect of Cl- on the induced potential observed in the control conditions appears to be due to an increase in Cl- permeability across an anion pathway. Flux data indicate that this pathway is transcellular.