Two types of strain rate change test are conducted. The strain rate reduction test is made for aluminium, copper, iron and niobium at strain rates up to about 20000/s. The attenuation measurement of the ultrasonic pulse superimposed upon the dynamic plastic deformation, a kind of strain rate change test with a high frequency perturbation of the strain rate, is made for aluminium and copper at strain rates up to about 10000/s. Results show that, in the above high strain rate range, the effect of the strain rate history upon the strain rate dependency of the flow stress is small.