The local electrical potential at individual grain boundaries and the potential distributions across polycrystalline samples have been measured by using scanning surface potential imaging with an atomic force microscope. Individual grain boundaries are isolated for measurement by micropatterning an array of contacts onto the surface of a ZnO-based varistor material. Quantification of the voltage dependence of the local voltage decrease and resistivity is illustrated. Comparisons are made by using optical and electron microscopy and spectroscopy. On a larger scale, potential distributions are mapped in a polycrystalline ZnO-NiO system that exhibits positive temperature coefficient of resistivity behavior.