The basal membrane of the retinal pigment epithelium (RPE) is the origin of two components of the electroretinogram, the fast oscillation and the light peak. Both of these responses originate from changes in basal membrane potential (Vba), and both are associated with changes in basal membrane resistance (Rba). In addition, many experimental manipulations that alter Vba also produce apparent changes in Rba. These findings raise the possibility that the basal membrane contains a voltage-sensitive conductance that operates in the physiologic range and is involved causally in light-evoked and other responses. We report the results of current clamp experiments on the isolated retina-RPE-choroid of chick that were designed to test for the presence of such a voltage-sensitive conductance in the basal membrane. Depolarizing Vba by 15 mV with retina-to-choroid current had essentially no effect on either the ratio of membrane resistances (Rap/Rba) or the transtissue resistance (RTotal), indicating no alteration in Rba. In contrast, hyperpolarizing Vba by 15 mV with choroid-to-retina current caused a gradual decrease in RTotal and increase in Rap/Rba. Analysis of accompanying changes in membrane voltages and changes in intracellular c-wave amplitude suggested that the most likely cause of the decrease in RTotal is a decrease in paracellular resistance. Voltage-sensitive conductances of the basal membrane appear to play little or no role in the resistance changes that accompany changes in Vba in the physiologic range. The conductance changes underlying the fast oscillation and light peak probably result from either the modulation of channels by second messengers or changes in intracellular ion concentration.