Recent analyses of transient creep data for lower crustal and upper mantle substances show that the parameters of the Burger's body rheological model, used up to now by geodynamicists, have much lower ratios of the short-term to long-term viscosities than the laboratory values. We have used these data to constrain the viscosity contrast across the 670 km discontinuity. Our results show that the previously inferred viscosity variations are now reduced by about a factor of ten in the new transient models. This finding is in agreement with the revised viscosity estimate based on long-wavelength geoid anomalies and seismic tomography. We find that time-dependent perturbations to the gravity field from recent ice movements are not at all small. ¿ American Geophysical Union 1987