  We present a convenient method for deriving the transformation of the dilaton
under T-duality in the path-integral approach. Subtleties arising in performing
the integral over the gauge fields are carefully analysed using Pauli-Villars
regularization, thereby clarifying existing ambiguities in the literature. The
formalism can not only be applied to the abelian case, but, and this for the
first time, to the non-abelian case as well. Furthermore, by choosing a
particular gauge, we directly obtain the target-space covariant expression for
the dual geometry in the abelian case. Finally it is shown that the conditions
for gauging non-abelian isometries are weaker than those generally found in the
literature.
