Internal heating and secular cooling are expected to decrease the geothermal gradient away from an adiabat. A scaling analysis that is quantified on the basis of published numerical models indicates that the interior geotherm should be nearly isothermal through the lower mantle if all heating is from within. For the more realistic condition that the mantle is partly heated from within and partly from below, we still expect the average thermal gradient to be significantly subadiabatic through much of the interior of the mantle. Values of the seismologically observable inhomogeneity parameter &eegr; ~ 1.04 to 1.07 should characterize the subadiabatic regions of the lower mantle. ¿American Geophysical Union 1987