The increase of the elastic compliance s11 of PLZT during cooling through Tc is discussed in terms of the model developed by Smolensky et al. for ferroelectrics with diffuse phase transition. The decrease of s11 and of the mechanical quality with increasing biasing field strength above Tc, as well as some peculiarities during polarization reversal below Tc, are explained by the appearance of the polar within the unpolar phase and vice versa, respectively.