The influence of Poisson"s ratio on overcoring was investigated using data from 47 tests. In these tests both the CSIRO HI triaxial cell and the triaxial cell developed by Swedish State Power Board were used. Boreholes drilled parallel to the major principal stress resulted in higher Poisson"s ratio compared to boreholes drilled at an angle to major principal stress. Numerical modelling by FLAC was used to simulate overcoring and biaxial testing. When the major principal stress is acting perpendicular to the borehole, the extensional strain is larger on the inner surface of the core during overcoring than when parallel to the borehole. Simulations of a biaxial test of a virgin core show extensional strain, which may be larger than critical extensional strain of the rock type, indicating non-elastic behaviour.