A nonlocal transport theory has been applied to solve some problems of high-rate deformation of solids. A dynamic stress-strain diagram is constructed which defines the elastic and plastic portions of the diagram from a unified viewpoint. The conditions for the accumulation of pulse stresses in relaxing media are determined. A mathematical model of exchange of momentum and energy between scale levels under high-rate deformation of solids is developed. A criterion of the instability initiation in transient plastic flow under shock loading are proposed. The instability criterion for high-rate deformation is verified by the example of shock loading of high-strength steel 30CrNi4Mo.