Relative travel time residuals for teleseismic P and PKIKP are used to determine the compressional velocity structure under the eastern Snake River Plain at Rexburg, Idaho. Damped least squares inversion of travel time residuals (modified from the method of Aki) indicates a large body of 3.5¿2.5% low relative velocity material centered under the northwest edge of the Snake River Plain. Less well resolved are high relative velocity bodies, one deep under the Idaho-Wyoming border, the other 300 km below the Idaho batholith. Comparison with similar velocity mappings of the Yellowstone region indicates that the snake River Plain low-velocity anomaly is an aged extension of the low-velocity body under the Yellowstone caldera. The upper half of the body has vanished in the eastern Snake River Plain, and the lower half is less anomalous than at Yellowstone. The strong dependence of mantle velocity structure on position under the Yellowstone-Snake River Plain volcanic system indicates loss of heat to the surface primarily by advective (heat movement by mass movement) mechanisms, not by conduction. No discrimination between propagating rift and upper mantle heat anomaly models of this northeast progressing volcanic system is possible because velocity structures associated with these hot spot models have not been predicted quantitatively. Upper mantle heat anomaly models (thermal plumes, chemical plumes, gravity anchors) explain the observes structure with fewer ad hoc assumptions, but propagating rift models cannot be eliminated.