The phenomenon of electrical degradation in ZnO varistors was studied by application of high‐intensity current pulses. A wave shape of 8×20 &mgr;s and rectangular waves of 1 and 2 ms were used. The degradation was estimated by reference electric‐field variation and by Schottky voltage barrier deformation. The results showed that current pulses reduce both the height and the width of the barrier voltage. It was also observed that the donor densityNddid not change but the surface states densityNsdecreased with degradation.