Measurements of the small-signal ac response of ZnO-based ceramic varistors are reported for frequencies f in the range 30<f<108 Hz and temperatures T in the range −200<T<300 °C. The high effective varistor dielectric constant results from the device microstructure with no detectable electrode contribution. The varistor leakage resistance ρp drops rapidly with frequency, a crude approximation being ρp∼1/ω. A broadened relaxation peak in the dissipation factor D=tanδ is observed at 3×105 Hz at room temperature. The peak has an exponential temperature dependence with an activation energy 0.36 eV.