Fleitout, L. Can the 1D viscosity profiles inferred from postglacial rebound data be affected by lateral viscosity variations in the tectosphere? In: Geophysical Research Letters, 28, 23 2001. 4403-4406 p. Abstract. We have tested the impact of lateral viscosity variations (LVV) in the top 200 km of the mantle (tecto-sphere) 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 sublitho-spheric mantle and a smaller thickness of the lithosphere than the synthetic model of viscosity.