Flat extruded wheat and rye bread was equilibrated to variable water activities using saturated salt solutions. At those water activities the bread was subjected to compression and three-point breaking tests. Compression test showed that water antiplasticizes both investigated breads. In the range of water activities from 0 to 0.53 for wheat bread, and from 0 to 0.59 for rye bread, the compression force reached 600 N at true strain 0.8. Compression proceeded in two steps; hence the mechanical resistance of the surface layers is larger than that of the center of the slice. Above the critical water activities water plasticizes the breads, and decrease of compression force with increasing water activity is recorded. Three-point breaking test shows that water affects mechanical properties of extruded breads in a complicated fashion. At water activities lower than 0.15, water plasticizes the material. At higher water activities antiplasticizing effect of water is evident. The critical water activities at which the plasticizing effect of water is evident are lower than those in compression test. The respective values are 0.49 and 0.44 for wheat and rye bread, respectively. Three-point breaking test is more sensitive to the kind of investigated material and shows larger differences between wheat and rye bread than the compression test. Critical water activities correspond to hydration levels at which internal dynamics of macromolecules begins. Hence, it is suggested that rotational and translational movements of macromolecules caused by adsorption of water are pronounced by the plastic deformation of the material. (c) 2005 Elsevier Ltd. All rights reserved.