  The effective Majorana mass which determines the rate of the neutrinoless
double beta decay, |<m>|, is considered in the case of three-neutrino mixing
and massive Majorana neutrinos. Assuming a rather precise determination of the
parameters characterizing the neutrino oscillation solutions of the solar and
atmospheric neutrino problems has been made, we discuss the information a
measurement of |<m>| > (0.005 - 0.010) eV can provide on the value of the
lightest neutrino mass and on the CP-violation in the lepton sector. The
implications of combining a measurement of |<m>| with future measurement of the
neutrino mass in tritium beta-decay experiments for the possible determination
of leptonic CP-violation are emphasized.
