Experimental results show that, when a specimen is subjected to vibration in the frequency range from 15 kc to 80 kc, the steady stress necessary to cause plastic deformation is appreciably reduced. This reduction is examined under different conditions of vibrational amplitude, frequency, strain, and temperature. Several possible explanations for this decrease in yield strength have been considered on the basis of dislocation theory. A discussion and a detailed picture of the responsible mechanism are included.