The discovered earlier unknown regularity of plastic behavior of viscous-elastic and viscous-plastic media, e.g. metals and alloys, in the conditions of their developed plastic strain consists in that when viscous-elastic and viscous-plastic media undergo vibration straining, their resistance to plastic strain reaches the smallest value when ratio of the frequency of such impact to the frequency of periodical elementary shifts in the plastic flow of above-mentioned media is within the frequency band of synchronization of these frequencies.