  We analyse the implications of right-handed neutrinos on the stability of the
electroweak vacuum in two-Higgs-doublet models with supersymmetry at high
scale. It is assumed that supersymmetry is broken at scale $M_S = 2 \times
10^{16}$ GeV and effective theory below $M_S$ is two-Higgs-doublet model of
type II with three generations of singlet neutrinos which induce small masses
for the standard model neutrinos through type I seesaw mechanism. We study the
high and low scale versions of seesaw mechanism. In both these cases, we show
that the presence of right-handed neutrinos significantly improves the
stability of electroweak vacuum if their Yukawa couplings with the SM leptons
are of ${\cal O}(1)$ or greater. However, this possibility is severely
constrained by the measured mass and couplings of Higgs and limits on the mass
of the charged Higgs from the flavour physics data. It is shown that the stable
or metastable electroweak vacuum and experimentally viable low energy scalar
spectrum require $\tan\beta \lsim 2.5$ and the magnitude of neutrino Yukawa
couplings smaller than ${\cal O}(1)$. The results obtained in this case are
qualitatively similar to those without right-handed neutrinos.

High Energy Physics - Phenomenology