Minimal extensions of the standard model that are motivated by grand u
nified theories or superstring models with an E6 symmetry can naturall
y predict heavy neutrinos of Dirac or Majorana nature. Such heavy neut
ral leptons violate the decoupling theorem at the one-loop electroweak
order and hence offer a unique chance for possible lepton-flavor deca
ys of the tau-lepton, e.g. tau --> eee or tau --> mumumu, to be seen i
n LEP experiments. We analyze such decays in models with three and fou
r generations.