Summary. The effect of the conduction electrons on the stopping power of metals has been considered from two essentially different points of view. No numerical calculations have been carried out, because we have found no completely satisfactory was of computing Z'e, the effective charge on the moving ion. A rather crude estimate made on the basis of the Thomas-Fermi atomic model indicates that Z'2/E is roughly constant. Using this relation one finds that the stopping power of the conduction electrons is of the same order of magnitude as the stopping power due to bound electron excitation, and is important for the greater part of the range of the particle.