Neutrino mass spectrum and mixing from neutrino oscillation data

Two schemes of mixing of four massive neutrinos with two close neutrino masses separated by a gap of Delta m^2 about 1 eV^2 can accommodate solar, atmospheric and LSND neutrino oscillation data. It is shown that long-baseline nu(e)->nu(x) and nu(mu)->nu(e) transitions are strongly suppressed in these schemes. The scheme of mixing of three massive neutrinos with a mass hierarchy that can describe solar and atmospheric data is also discussed. It is shown that in this scheme the effective Majorana mass that characterizes the matrix element of neutrinoless double-beta decay is smaller than about 10^{-2} eV.