The compatibility of mantle Urey ratios Um calculated from thermal evolution models with those calculated from geochemical models is considered. The initial heat generation rate in the mantle used for the thermal evolution models was derived from two geochemical models of the present day primitive mantle with concentrations of U of 220 ppb (2.7 times chondritic) and 26 ppb (3.5 times chondritic), respectively. The depletion of the mantle through continental crust production was allowed for; the effect of including the continental crust on Um is a reduction of its value by about 0.1. To balance the observed average value of the surface heat flow qs was assumed that the core may contain K up to cosmochemically constrained quantities. For 26 ppb U in the present primitive mantle no core heat sources are required. The present value of Um for this model is 0.57 and the core heat flow is about 12% of qs. With 20 ppb U in the present primitive mantle, heat production equivalent to one or two silicate earth budgets of K is required in the core to balance qs and Um is between 0.35 and 0.4. The core heat flow is then between 25% and 33% of qs. ¿ American Geophysical Union 1993