We analyze the conditions that allow the bulk-grain regions of a polycrystalline semiconductor to be explored through electrical measurements. The temperature dependence of grain resistivity in ZnO varistors (300–430 K) is presented, and a positive temperature coefficient is found. This is consistent with a free-carrier density approaching exhaustion, and an electron mobility controlled mainly by lattice (both optical and acoustical) scattering. No grain conductivity activation energy is to be found above room temperature and, therefore, ac-impedance techniques can be inadequate for the evaluation of grain conductivity and shallow donor activation energy.