It was observed that the dielectric permittivity of PbMg1/3Nb2/3O3 (PMN) relaxer ceramics increases with increasing amplitude of the applied ac field. A strong nonlinearity occurs at temperatures where the frequency dispersion of the dielectric response is observed. A similarity was found between the effects of the amplitude and frequency on the permittivity. It was shown that, taken together, these data evidence that the relaxation phenomena in PMN are controlled by domain-type dynamics rather than thermally activated flips of the local spontaneous polarization.