An apparatus is described for the measurement of internal friction under conditions of hydrostatic pressure transmitted by a liquid. The material to be tested is in the form of a helical spring. The motion of this spring under the influence of an axial sinusoidal force is compared to the motion of a reference spring made of a material exhibiting negligibly small losses under the influence of the same force. The phase angle between stress and strain and thus the internal friction in the specimen can be determined by this procedure. External damping due to viscous interaction between the specimen surfaces and the pressure transmitting fluid is kept inappreciably small by employing very low frequencies of vibration (in the order of 1 cycle/min and less). There is no theoretical limit to the lowest frequency that can be used. Strain amplitudes of 10−7−10−5have so far been realized; values beyond these limits are possible. A simple theoretical analysis of the behavior of the test arrangement is discussed. Some experimental results are presented.