Varistors are made of specially doped, semiconducting zinc oxide. There are potential barriers at the grain boundaries of the material that are formed due to electrons that occupy acceptor states. At small voltages only few electrons can cross these barriers and the varistor exhibits a high resistance. Above the so-called breakdown voltage, electron-hole pairs are generated by impact ionization. The generated holes lower the barriers at the grain boundaries causing a highly non-linear decrease of the materials resistance. This so-called varistor effect makes varistors capable of being used for surge protection of electronic devices or circuits. For this purpose the varistor is connected in parallel to the circuit to be protected. The characteristic breakdown voltage is determined by the effective number of grains in series. This work deals with the influence of mechanical load on the conductivity of ZnO varistors. Since ZnO exhibits a pronounced piezoelectric effect, one might expect that mechanical stress generates polarization charges in the material that influence the potential barrier heights and as a consequence the I-V-characteristics of the varistor. Compressive tests with uniaxial application of force to the specimen in combination with I-V-measurements with forward and reversed bias have been carried out. The tests have been performed on bulk-specimens with approximately 130 grains in series and multilayer-varistors with only 2-3 grains in series. It has been found that the current density in the bulk-specimens strongly increases with increasing applied pressure for forward and reversed bias. Surprisingly, the tests on the multi-layer varistors have shown that the conductivity of these components can either increase or decrease. The natural asymmetry for forward and reversed bias can even increase.