Analysis of the conservation equations demonstrates that the ratio of fluid-to-temperature velocities is a constant, even in heterogeneous media, Because tracers move at the bulk fluid velocity, tracer test analysis can be used to predict temperature declines at production wells that result from injection. Tracer histories are transformed into predicted temperature histories through a simple variable transformation. Examples demonstrate that the method works well for single-phase flow in heterogeneous (but porous) media. In the presence of strong permeability correlation (layering), the method's accuracy degrades somewhat; the onset of cooling, however, is still predicted reasonably accurately.