Fluctuation in transepithelial current was analysed in skins of Rana esculenta and Bufo viridis. Perturbation of the tissues to serosa + 40 mV or + 80 mV activated a pathway for conductive C1 transport and led to the appearance of a Lorentzian component in the power density spectrum. The presence of C1 on the mucosal side was mandatory for the observation of Lorentzians. Corner frequencies, fc, ranged between 48 Hz and 102 Hz. The mean value was slightly larger at 40 mV than at 80 mV (88.3 _ 4.4 versus 72.5 + 6.4 Hz). Plateau values, So, were proportional to the magnitude of the activated C1 current. Single-channel currents of 48.3 _ 5.2 fA and 93.0 + 8.7 fA were calculated at 40 mV and 80 mV, respectively, with the assumption of equal open and closed probabilities of the channels. From these data, a voltage-independent open-channel conductance of about 1.2 pS is obtained. Activation of C1 conductance was associated with an increase in channel density. Stimulation of C1 conductance by procaine did not affect So and fc despite considerable stimulation of transepithelial C1 current. This could indicate that non-gated channels participate in the voltage-sensitive C1 conductance. Mucosal application of inhibitors of C1 conductance (3',5-dichlorodiphenylaminocarboxy acid, MK-196) decreased So in a concentration-dependent manner, but had no effect on fr The data could support the hypothesis that the voltage-sensitive transepithelial C1 transport is localized to a cellular compartment, which is most likely the mitochondria-rich cells. The possibility cannot be excluded, on the other hand, that the fluctuation in current originates from highly organized, specific sites in the junctional complexes.