If (as is suggested by both observation and laboratory studies) mantle degassing of 3He occurs primarily with melt formation, then the 3He/4He ratios and 3He fluxes observed in continental extensional basins (e.g., Basin and Range, Pannonian Basin, Rhine Graben) require the input of magma from the mantle. The processes governing the relations between heat and helium fluxes associated with magmatic processes are reviewed and evaluated with respect to simplified mechanism of enhanced heat flow in extensional basins. The observed heat flux enhancements from underplating (large-scale accretion of a melt to the base of the crust) that might accompany extension would require large fluxes of 3He and very high 3He/4He ratios to occur in characteristic crustal fluids with a time delay of hundreds of millions of years. The mapping to date of continental 3He/4He in characteristic crustal fluids suggests that this is not the case; high 3He/4He ratios and high 3He fluxes are not dominant in older inactive regions. Furthermore, since high 3He/4He ratios are observed in Cenozoic extensional basin, it is suggested that (1) minor mantle magma intrusion to shallow levels in the crust is responsible for the (small) 3He fluxes observed in extensional terranes on a geologically short time scale and (2) stretching, lateral flow of lower crustal material, erosion, and/or other mechanisms are responsible for the bulk of the higher heat flow (and isostatic compensation) in extensional basin. Combined geophysical and geochemical studies of the 3He flux in extensional basins of all geologic ages and measurements of the effective transport of helium in the crust will better define the role for, e.g., stretching, magmatism, underplating, and lateral inflow during continental extensional tectonics. ¿ American Geophysical Union 1993