  Recent experimental results suggest that the neutrinos of the Standard Model
are massive, though light. Therefore they may mix with each other giving rise
to lepton flavour or even lepton number violating processes, depending on
whether they are Dirac or Majorana particles. Furthermore, the lightness of the
observed neutrinos may be explained by the existence of heavy ones, whose
effects on LFV would be very sizeable. We present an analysis of the effect of
massive neutrinos on flavour-changing decays of the Z boson into leptons, at
the one-loop level, independent of neutrino mass models. Constraints from
present experiments are taken into account.
