Skin from larval bullfrogs was mounted in an Ussing-type chamber in which the apical surface was bathed with a Ringer solution containing 115 mM K⁺ and the basolateral surface was bathed with a Ringer solution containing 115 mM Na⁺. Ion transport was measured as the short-circuit current (I_sc) with a low-noise voltage clamp, and skin resistance (R_m) was measured by applying a direct current voltage pulse. Membrane impedance was calculated by applying a voltage signal consisting of 53 sine waves to the command stage of the voltage clamp. From the ratio of the Fourier-transformed voltage and current signals, it was possible to calculate the resistance and capacitance of the apical and basolateral membranes of the epithelium (R_a and R_b, C_a and C_b, respectively). With as the anion, R_m decreased rapidly within 5 min following the addition of 150 U/ml nystatin to the apical solution, whereas I_sc increased from 0.66 to 52.03 µA/cm² over a 60-min period. These results indicate that nystatin becomes rapidly incorporated into the apical membrane and that the increase in basolateral K⁺ permeability requires a more prolonged time course. Intermediate levels of I_sc were obtained by adding 50, 100, and 150 U/ml nystatin to the apical solution. This produced a progressive decrease in R_a and R_b while C_a and C_b remained constant. With Cl as the anion, I_sc values increased from 2.03 to 89.57 µA/cm² following treatment with 150 U/ml nystatin, whereas with gluconate as the anion I_sc was only increased from 0.63 to 11.64 µA/cm². This suggests that the increase in basolateral K⁺ permeability produced by nystatin treatment, in the presence of more permeable anions, is due to swelling of the epithelial cells of the tissue rather than the gradient for apical K⁺ entry. Finally, C_b was not different among skins exposed to Cl, , or gluconate, despite the large differences in I_sc, nor did inhibition of I_sc by treatment with hyperosmotic dextrose cause significant changes in C_b. These results support the hypothesis that increases in cell volume activate K⁺ channels that are already present in the basolateral membrane of epithelial cells.