  We consider $ \nu_{\mu} \to \nu_{\tau} $ oscillations under the assumption
that there is a see-saw type mixing of the light neutrinos with heavy Majorana
particles. It is shown that the existing data, including the recent LEP data,
do not exclude the possibility that the additional terms in the transition
probability due to this mixing could be of the same order of magnitude as the
usual oscillating term. Detail investigations of $ \nu_{\mu} \to \nu_{\tau} $
transitions in future CERN and Fermilab experiments could allow to get
informations not only about the neutrino masses and mixing but also about the
mixing of neutrinos with heavy Majorana particles.
