We analyze the possibility of universality violation in diagonal lepto
nic decays of the Z boson, in the context of interfamily ''seesaw'' mo
dels. In a minimal extension of the standard model with right-handed n
eutrino fields, we find that universality-breaking effects increase qu
adratically with the heavy Majorana neutrino mass and may be observed
in the current experiments at the CERN e+e- collider LEP.