The influence of external currents on the potential response of polymeric membrane ion-selective electrodes (ISEs) is analyzed theoretically. The treatment is based on the well-established potential model of ISE membranes, extended by contributions from current flow and corresponding electrodialytic ion fluxes through the membrane. A general implicit solution is derived that describes the potential response in terms of the external current, the selectivity characteristics of the membrane, and the compositions of the membrane, the sample solution, and the inner solution, respectively. Explicit relationships are given for limiting cases, e.g. when only one kind of cation is present in the solution or when the disturbance by interfering cations is small. For the latter case, theoretical curves are compared to experimental ones and are shown to give a good correspondence.