We have carried out numerical simulations to study the average temperature and thermal structure of an internally heated mantle. It is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle, whereas in a compressible mantle they occur over laterally extensive regions near the core-mantle boundary (CMB). For chondritic concentrations of heat-producing elements (hpe's), the average thermal conductivity in the mantle must be high to avoid heating the core, and the temperature of the CMB (TCMB) is low to avoid large-scale melting. The mantle may have been extensively molten in the Archaen. ¿ American Geophysical Union 1989