After plastic deformation at high confining pressure, marble and limestone show changes of length during release of the pressure that differ markedly from the normal elastic changes with pressure in undeformed specimens; after compression, the specimens tend to elongate much more than undeformed specimens, while after extension, the length changes tend to be less than in undeformed specimens. These departures from normal elastic behaviour, called here secondary length changes, are a function of the changes of pressure, not of time. During a subsequent pressure cycle, they are only partly reversible. They are accompanied by a decrease in density. The absence of such secondary length changes in polycrystalline copper and in single crystals of calcite leads to the hypothesis that they have their origin in intergranular stresses, which give rise to grain distortion and the opening of internal fractures. The chief sources of intergranular stresses in calcite aggregates are the anisotropy of yield stress of the grains, leading to residual stresses after the deformation, and the anisotropy of elastic linear compressibility of the grains, leading to further internal stresses being superposed on the former as the pressure falls. Similar secondary length changes have also been observed in granite, sandstone and serpentinite after deformation in compression, although the magnitudes of the effects are somewhat less than in the calcite rocks, especially in the case of the sandstone. These effects can probably also be attributed to intergranular stresses, especially in the granite where intergranular stresses can arise from the differences of properties between minerals. It is not clear whether such effects are to be expected in nature but they could be of importance in facilitating phenomena involving diffusion.