ABSTRACT: The motivation for this study has been to establish a relation between static and dynamic Poisson's ratio based on sonic measurements. The work was based on laboratory tests where both static moduli and sound velocities were measured simultaneously. The results showed that the static and dynamic Poisson's ratio behaved differently with increasing shear stress. We also found indication of creep effects on the Poisson's ratio. Based on the formalism established by Fjær (1999) a relation between static and dynamic Poisson's ratio was established, which describes the relation during the entire strength range for a standard rock mechanical tests. The relation depends on the key elements used to describe the relation between the static and dynamic values for bulk modulus and Young's modulus, plus an additional term explicitly associated with dilatancy. The equation was found to give a good description of how the Poisson's ratio develops during the test. However, the model occasionally overpredicts the value.