  We consider radiative corrections to the decay rate $\Gamma(H\rightarrow ZZ)$
of the heavy {\it CP}-even Higgs boson of the minimal supersymmetric model to
two $Z$ bosons. We perform a one loop Feynman diagram calculation in the
on-mass-shell renormalization scheme, and include the third generation of
quarks and squarks. The tree level rate is suppressed by a mixing angle factor
and decreases as $1/M_H$ for large $M_H$. The corrected rate overcomes this
suppression and increases with $M_H$ for $M_H > 500$~GeV. The corrections can
be very large and depend in detail on the top squark masses and $A$-term, as
well as the supersymmetric Higgs mass parameter $\mu$.
