  The smallness of the neutrino masses can be well understood within the seesaw
mechanism. We analyse two cases of the minimal extension of the standard model
when one or two right-handed fields are added to the three left-handed fields.
A second Higgs doublet is included in our model. We calculate the one-loop
radiative corrections to the mass parameters which produce mass terms for the
neutral leptons. In both cases we numerically analyse light neutrino masses as
functions of the heavy neutrinos masses. Parameters of the model are varied to
find light neutrino masses that are compatible with experimental data of solar
$\Delta m^2_\odot$ and atmospheric $\Delta m^2_\mathrm{atm}$ neutrino
oscillations for normal and inverted hierarchy.
