[en] The contribution from the irreversible displacement of non-180 deg domain walls to the direct longitudinal piezoelectric d₃₃ coefficient of BaTiO₃ and Pb(Zr, Ti)O₃ ceramics was determined quantitatively by using the Rayleigh law. Effects of the crystal structure and microstructure of the ceramics as well as the external d.c. pressure on the domain wall contribution to d₃₃ were examined. In barium titanate, this domain wall contribution is large (up to 35% of the total d₃₃, under the experimental conditions used) and dependent on the external d.c. pressure in coarse grained ceramics, and much smaller and independent of the external d.c. pressure in fine-grained samples. The presence of internal stresses in fine-grained ceramics could account for the observed behaviour. The analysis shows that the domain-wall contribution to the d₃₃ in lead zirconate titanate ceramics is large in compositions close to the morphotropic phase boundary that contain a mixture of tetragonal and rhombohedral phases, and in rhombohedral compositions (up to 40% of the total d₃₃). The domain-wall contribution to the piezoelectric response from the irreversible displacement of domain walls is significantly smaller in tetragonal Pb(Zr, Ti)O₃ samples where it decreases with increasing spontaneous strain. (author)