We study the effects of a possible new heavy neutral vector boson ZH on the partial and total widths of the ZL which are being studied at LEP. The analysis is formulated in as general terms as possible, without commitment to a particular model. The constraints on from CDF and the new data on and are taken into account. As expected, the induced deviations are at the level of a few per cent. The allowed range decreases with increasing mt. The lepton widths and the peak cross-sections are particularly sensitive. A fourth light neutrino is excluded also in extended gauge models.