Polarization switching in (001)-oriented Pb(Mg1/3Nb2/3)O-3-x%PbTiO3, PMN-x%PT, crystals for x=20 and 35 has been investigated in situ by scanning force microscopy in a piezoresponse mode, under a step-increased dc electrical voltage (V-dc). The results demonstrate that switching initiates by the nucleation of nanosized polar regions with a reversed polarization. For x=20, nucleation occurs heterogeneously throughout the volume of the crystal. However, with increasing PT content, nucleation tends to preferentially initiate near preexisting twin boundaries; with increasing V-dc, the domains becoming increasingly "diffuse" due to heterogeneous nucleation and growth throughout the crystal. (C) 2004 American Institute of Physics.