All the possible schemes of neutrino mixing with four massive neutrino
s inspired by the existing experimental indications in favour of neutr
ino mixing are considered. It is shown that the scheme with a neutrino
mass hierarchy is not compatible with the experimental results, likew
ise all other schemes with the masses of three neutrinos close togethe
r and the fourth mass separated by a gap needed to incorporate the LSN
D neutrino oscillations. Only two schemes with two pairs of neutrinos
with close masses separated by this gap of the order of 1 eV are in ag
reement with the results of all experiments. We carefully examine the
arguments leading to this conclusion and also discuss experimental con
sequences of the two favoured neutrino schemes.