1. Basolateral membranes of the frog urinary bladder were investigated after increasing the cationic conductance of the apical membrane by the incorporation of nystatin. 2. K+ currents were recorded in the presence of a mucosa to serosa oriented K+ gradient (SO4(2-) Ringer solution). Nystatin caused a rapid rise of the short-circuit current (Isc) followed by a slow increase over a period of 1-2 h. 3. Impedance analysis showed that the apical membrane resistance was drastically reduced by nystatin. The slow increase in Isc was accompanied by a progressive increase in basolateral conductance. 4. The transepithelial current and conductance recorded in the presence of nystatin could be depressed with lidocaine added to the mucosal and serosal solution. The effects of lidocaine were completely reversible. 5. Noise analysis showed that lidocaine induced additional fluctuations in Isc. The spectrum of these fluctuations was of the Lorentzian type. This noise component is caused by the random interruption of the current through the basolateral K+ channels. The Lorentzian parameters were used to calculate the microscopic parameters of the basolateral K+ channels.