North American horizontal post-glacial rebound velocities are computed for three five-layered, incompressible, spherical Maxwell earth models, using a simple glacial loading cycle centered on Hudson Bay. The reference earth model has a 120-km thick elastic lithosphere and upper- and lower-mantle viscosities of 1021 Pa⋅s and 2¿1022 Pa⋅s, respectively. The second model has a lower viscosity upper-mantle (5¿1020 Pa⋅s) and a higher viscosity lower-mantle (1023 Pa⋅s), and the third has a 250-km thick lithosphere. The reference model and the low-viscosity upper-mantle model calculations are in good agreement with uplift and gravity data, although they may somewhat overestimate the present-day central uplift rate. The thick lithosphere model fares less well because it fails to exhibit the well-pronounced peripheral bulge sinking that is required by eastern North America tide gauge data. The reference model generally has the largest horizontal rebound velocities, reaching a maximum of 4 mm/yr towards Hudson Bay at 2400 km from the load center. The effect of a thick lithosphere or of a low-viscosity upper-mantle is to reduce present-day horizontal velocities, especially in the formerly glaciated region. The sensitivity of horizontal motions to lithospheric thickness and upper-mantle viscosity may assist in discriminating between different models of the earth's rheology once sufficient high-quality geodetic data exist. ¿ American Geophysical Union 1990