Schemes of neutrino mixing from the results of neutrino oscillation experiments

The mixing of three and four massive neutrinos is considered. It is shown that the neutrino oscillation data are not compatible with a hierarchy of couplings in the three-neutrino case. In the case of four neutrinos, a hierarchy of masses is not favored by the data. Only two schemes with two pairs of close masses separated by a gap of the order of 1 eV can accommodate the results of all experiments. If the existing indications in favor of neutrino oscillations will be confirmed, it will mean that the general features of neutrino mixing are quite different from those of quark mixing.