Barriers of very different character such as localized obstacles and the extended Peierls- Nabarro relief may control the dislocation motion in crystals in different stress and temperature ranges. The great difference in the microscopic parameters characterizing these two mechanisms, e.g., in the activation volumes, manifests itself even on the macroscopic scale as a strong change of the plastic properties in a rather narrow transition range of the temperature. A theory describing the temperature dependence of the flow stress and the strain rate sensitivity near the transition has: been developed and compared with experimental data on the plastic deformation of cubic ZrO2 single crystals in a soft orientation.