  The radiative corrections to the $3\times3$scalar and the $2\times2$
pseudoscalar neutral Higgs boson massmatrices are calculated in the
supersymmetric extension of the standard model including a gauge singlet
superfield in the effective potential approach. The full $t$ and $b$
quark/squark contributions including the nonlogarithmic terms are taken into
account, which are seen to affect the result significantly. An analytic formula
for the upper bound on the mass of the lightest neutral Higgs scalar including
these corrections is given. (Figure available upon request)
