  We consider the lepton sector of the Standard Model and allow for an
arbitrary number of Higgs doublets and, moreover, for the presence of
right-handed neutrino singlets which enable the seesaw mechanism. In this
framework, we identify and calculate the dominant one-loop radiative
corrections to the tree-level mass matrix of the light neutrinos. The
interesting feature is that both the tree-level and the one-loop contributions
to the light-neutrino mass matrix are quadratic in the Yukawa couplings, with
the effect that the one-loop contribution is smaller than the tree-level one
mainly because of the one-loop factor $(16\pi^2)^{-1}$. We also point out the
possibility of generating radiatively--in this framework--the ratio of solar
over atmospheric neutrino mass-squared differences, as needed for the
large-mixing-angle MSW solution of the solar-neutrino problem.
