The polydispersive dielectric response of the relaxor ferroelectric lead magnesium niobate was investigated using nonresonant spectral hole-burning experiments. Using large alternating electric fields, specific parts of the relaxation-time distribution could be addressed selectively. This provides evidence for the dynamic heterogeneity of the dipolar reorientation process. The refilling of single as well as of double holes was much slower than the time scale set by the pump frequencies. Upon refilling, the holes exhibited no signs of spectral broadening. The present findings suggest a speedup of polarization response associated with a domain-wall depinning that is induced by the hole-burning event.[S0031-9007(98)07584-X] PACS numbers: 77.84.Dy, 77.22.Ej, The relaxation of disordered and glass-forming materials was often pictured as being due to independently responding nanoregions. Only recently this notion could be substantiated experimentally for supercooled liquids and polymers [1,2]. Above the glass transition the time scale characterizing this heterogeneity was found to be very close to that of the structural relaxation itself. Thus the heterogeneity, which gives rise to the ubiquitously observed broadening of susceptibility spectra here, is dynamic in nature. This may have been expected since the notion of a static heterogeneity is incompatible with the concept of a metastable ergodic liquid state. On the other hand, in polar, disordered crystals, such as relaxor ferroelectrics (RF), transmission electron microscopy has revealed clear evidence for nanoscale ͑ϳ5 nm͒ heterogeneity [3]. It arises from local fluctuations in the chemical composition of these highly polar materials, and typically is quasistatic near and below room temperature. However, there is evidence from computer simulation studies dealing with spin glasses [4] suggesting that dynamic heterogeneity may also be found in crystalline substances. In fact, models consistent with the existence of dynamic heterogeneity have been successful in describing the polydispersive dielectric response of multidomain ferroelectrics [5].In this Letter we report direct experimental evidence for dynamic heterogeneity in a relaxor ferroelectric, i.e., in a crystal, via the technique of dielectric nonresonant spectral hole burning (NHB). This method is based on a pump, wait, and probe scheme [6]. In contrast to NHB results previously obtained for supercooled liquids, we find that in RF the hole refilling proceeds slow on the time scale that governs the dynamics of the mesoscopic regions addressed during the frequency-selective pump. Obviously, the existence of well-established static heterogeneities in RF promotes the slowdown of spectral reequilibration.For our studies we have used PbMg 1͞3 Nb 2͞3 O 3 (PMN) ceramics doped with 10% PbTiO 3 (90PMN-10PT) which has been found to be much less susceptible to chemical aging than "pure" PMN [3]. PMN is one of the prototypic relaxors [3], and the polydispersive complex dielectric constant of this material has been document...