An apparatus is described in which internal friction (Q-1) can be measured for specimens of dimensions 0.01 × 2 × 15 mm3 during the process of plastic deformation in the temperature range from 4.2 to 300 K and the frequency range 102 to 104 Hz. The cryostat has three independent helium spaces, allowing Q-1 to be measured in the magnetic field of a superconducting solenoid. The relative error in measuring Q-1 during plastic deformation is ≤5% at a deformation rate of 10-3. © 1979.