The influence of heat treatment in lead zirconate titanate Pb(Zr,Ti)O-3 (PZT) ferroelectrics on the domain structure and strain behavior has been investigated. Temperature-induced changes in the domain configuration, for example, appearance of nanodomains within microdomains, were observed by in situ hot-stage transmission electron microscopy. During the transient post annealing state the strain behavior under electric field differed from that of the nonheat-treated ferroelectrics. Directly after cooling the difference was most pronounced and decreased as a function of time. The enhanced response to an electric field of 4kV/mm resulted in qualitatively different effects on strain for PZT from the tetragonal side and from the rhombohedral side of the morphotropic phase boundary.