For the antiferroelectric crystals PbZrO3 and PbHfO3 pure and doped with small amount of PbTiO3 the measurements of electrostrictive strain e have been performed in paraelectric phase as a function of temperature, frequency (40-400 Hz) and strength (100-700 kV/m) of an applied electric field. In particular, compliance with quadratic relation between strain e and the electric field E (e = ME2) in different frequency regions was examined. The observed effects for PbZrO3 and PbHfO3 were compared and discussed with previously investigated low frequency dielectric dispersion found in those antiferroelectric materials. The experiments showed that in the frequency range where the dipolar relaxation exists, the departure from the quadratic relation (e = ME2) can be clearly observed for PbZrO3 and PZT single crystals. In all investigated single crystals (PbZrO3, PZT and PbHfO3) the dependence of strain versus electric field obeys the electrostrictive relation in the frequency range below and above dielectric relaxation observed. For frequencies beyond dielectric dispersion the electrostrictive coefficients Q(11) for all samples are temperature independent and take values typical for ferroelectric and antiferroelectric materials (i.e. Q(11) congruent to 2 x 10(-2) - 3 x 10(-2) m(4)/C-2).