We report on the experimental investigation of a high-frequency (1 MHz-1.8 GHz) dielectric dispersion in unpoled and poled Pb(Zr,Ti)O-3 ceramics. Two overlapping loss peaks could be revealed in the dielectric spectrum. The linear dependence between the lower-frequency peak position and average grain size D, which holds for D/<= 10 mu m, indicates that the corresponding polarization mechanism originates from piezoelectric resonances of grains. The intensity of the higher-frequency peak is drastically reduced by poling. It is thus proposed that this loss peak is related to domain wall contribution to the dielectric dispersion.