[en] A theory is outlined which is capable of explaining the observed effects of neutral-beam injection on impurities in tokamak plasmas. In the collisional regime, it is shown for a representative model problem that for sufficiently strong co-injection the outward impurity flux component produced by the radial electric field becomes larger than the inward flux components due to the density gradient, rotation and direct beam-impurity momentum exchange, thereby producing an outward impurity flux. In the mixed regime, a preliminary version of the theory provides a reasonable interpretation of the PLT impurity flow reversal experiments. This preliminary theory, when adjusted to match the PLT results, predicts significant impurity flow reversal in TFTR and in a plasma of the size of FED and INTOR