Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict also new charged fermions. A mixing of the known fermions with new states with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree level flavour changing neutral interactions mediated by Z exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour changing neutral currents are mainly due to the exchange of the lightest new neutral gauge boson Z'. We show that the present experimental limits on μ-e conversion in nuclei give a nuclear-model-independent bound on the Z-e-μ vertex which is twice as strong as that obtained from μ --> eee. In the case of E6 models these limits provide quite stringent constraints on the Z' mass and on the Z-Z' mixing angle. We point out that the proposed experiments to search for μ-e conversion in nuclei have good chances to find evidence of lepton flavour violation, either in the case that new exotic fermions are present at the electroweak scale, or if a new neutral gauge boson Z' of E6 origin lighter than a few TeV exists.One of us (D.T.) acknowledges a post-doc fellowship from the Spanish Ministry of Education and Science. This work has been partially supported by CICYT (Spain), under grant AEN/90-0040.Peer reviewed