  We consider an extension of the Standard Model by right-handed neutrinos and
we argue that, under plausible assumptions, a neutrino mass of ${\cal
O}(0.1)\,{\rm eV}$ is naturally generated by the breaking of the lepton number
at the Planck scale, possibly by gravitational effects, without the necessity
of introducing new mass scales in the model. Some implications of this
framework are also briefly discussed.
