Variation in radiogenic heat production of rocks of diverse magma series (representative of calcic, calc-alkalic, alkali-calcic, and alkalic petrographic provinces on a worldwide basis) is better correlated with some form of magmatic differentiation index, rather than simply with potassium content alone as commonly supposed. The “lime-alkali” (Peacock) index generally gives the best correlation of the various indices tested. Plots of heat production for igneous suites versus indices of differentiation (“heat production trends”) demonstrate systematic correlations of heat productivity as functions of fractionation trends and/or spatial-temporal associations. In general, heat production trends of calcic suites tend to indicate lower radiogenic heat productivity than those of more alkalic suites for a given silica or potassium content. Because many recent measurements of heat flow in plutons can be linearly related to heat production of surface rocks, these “heat production trends” have important bearing on the interpretation of crustal heat flow. Our data suggest possible alternative interpretations of the heat production-heat flow pattern for the Sierra Nevada which differ from that in Lachen-bruch's (1968) preliminary geothermal model.