Observations indicate that the orientation of the stress field in Eastern Canada has changed significantly during the last 9,000 years. If this temporal rotation of stress orientation is due to changes of the dominant stress component from rebound stress 9,000 years ago to tectonic stress at the present, then this rotation in stress orientation can be used to constrain mantle viscosity and the difference between the horizontal tectonic (principal) stresses. Rebound stress in a Maxwell earth due to the application of a realistic ice load is calculated with the Finite Element method and is superposed on tectonic stress to give the total stress. It is demonstrated that large tectonic stress differences (>10 MPa) or high viscosity (≥1022 Pa-s) in the lower-mantle cannot explain this temporal stress rotation because in the former case, stress orientation is determined by the static tectonic stress while for the latter case, rebound stress continues to dominate the current stress orientation. ¿ American Geophysical Union 1996