We have tested the impact of lateral viscosity variations (LVV) in the top 200 km of the mantle (tectosphere) on the interpretation of Fennoscandian postglacial uplift data in terms of 1D viscosity models. Our test consists of two steps. In the first step, we simulate the postglacial uplift for a given 2D viscosity structure involving a cratonic lithospheric root below the ice sheet and an asthenosphere below oceanic areas. In the second step, we interpret the result of this simulation in terms of a 1D viscosity model. The resultant 1D viscosity model shows a lower viscosity in the lower mantle, a higher viscosity in the shallow sublithospheric mantle and a smaller thickness of the lithosphere than the synthetic model of viscosity. ¿ 2001 American Geophysical Union