The dependence of the permittivity, on uniaxial pressure of ceramics in the paraelectric, ferroelectric and relaxer phases has been investigated for BaTiO3, Pb(ZrTi)O-3 and Pb(Mg1/3Nb2/3)O-3. In the paraelectric ic phase, upon the application of stress, a reversible decrease in the permittivity is observed. In the ferroelectric phase, the observed behaviors can be divided in two classes depending on the type of ionic defects present in the ceramics. Both behaviors are characterized by an instantaneous increase in the permittivity at both stress application and release (Delta sigma) but they differ in the following relaxation which can be a decrease (type A) or an increase (type B) in the permittivity. In relaxer materials the behaviors observed are similar to the paraelectric and ferroelectric phases as the temperature is lowered. The mechanisms of the different observed behaviors are discussed. (C) 1999 Published by Elsevier Science Limited. All rights reserved.