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Intracellular micro-electrode techniques were used to measure the electrical resistances of the cell membranes and the shunt pathway and intracellular ionic activities in trout urinary bladder when the tissue was incubated in Ringer solution and in the presence of the polyene antibiotic ionophore amphotericin B. In control conditions the transepithelial potential was zero and the intracellular potential was -56 mV. The intracellular ionic activities measured with single- and double-barrel ion-sensitive micro-electrodes for the first time in a fish bladder (aiNa = 16 mM, aiK = 87 mM, and aiCl = 21 mM) indicate an active accumulation of K and Cl ions and an active extrusion of Na ions by the cell. The maintenance of intracellular Cl activity above its equilibrium value depended on the presence of Na ions in the mucosal medium, but was independent of the presence of K ions. Flat cable analysis yielded values for transepithelial, apical, basolateral and shunt resistances of 197, 2790, 1986 and 205 omega cm-2 respectively. Equivalent circuit analysis using amphotericin B yielded similar values for shunt resistance. The paracellular pathway accounts for 96% of transepithelial current flow and this epithelium may be classified as 'leaky'. The cells are electrically coupled with a space constant of 354 micron. Amphotericin B when added to the mucosal solution induced an immediate serosa positive transepithelial potential of about 9 mV and a short-circuit current of 64 microA cm-2. The Vt was ouabain sensitive and dependent on mucosal Na concentration. The origin of the antibiotic induced transepithelial potential was an increase in the sum of the cell membrane electromotive forces. The apical membrane potential depolarized to -7 mV and its resistance fell to 433 omega cm-2. During the first 10 min of exposure aiNa increased to 80 mM and aiK decreased to 7 mM with only a small change in aiCl. The changes in cellular Na+ and K+ activities were in accordance with their passive redistribution down their electrochemical gradients.