The electroluminescence of Dy3+ and sm(3+) ions inserted in polycrystalline semiconducting zinc oxide was studied using a rare earth-doped sintered zinc oxide disc sandwiched between two metallic solders. This device, which corresponds to a metal-semiconductor-metal (MSM) structure, is similar to a varistor and is able to operate under either ac or de mode. The excitation of the trivalent rare earth ions is a consequence of a hot electron impact process. The tare earth ions are located near the grain boundaries. Their luminescence acts as a probe and evidences hot electrons near the grain boundaries where, in accordance with the model of varistors, they are produced by thermionic emission.