The determination of mantle viscosity conditions from observations of relative sea level changes associated with glacial unloading of the Laurentide and Fennoscandia ice sheets has been reexamined. The conclusion is reached that the adjustment has been an upper mantle process associated with the low-seismic-velocity, low-viscosity asthenospheric layer rather than a whole mantle process of essentially unfirom mantle viscosity. The critical data for resolving this dichotomy are those from the zone of peripheral subsidence adjacent to the region of central uplift of the Laurentide and Fennoscandia ice sheets. ¿ American Geophysical Union 1988