  We study the incorporation of QCD effects in the basic electroweak
corrections \drcar, \drcarw, and \dr. They include perturbative
\Ord{\alpha\alpha_s} contributions and $t\bar{t}$ threshold effects. The latter
are studied in the resonance and Green-function approaches, in the framework of
dispersion relations that automatically satisfy relevant Ward identities.
Refinements in the treatment of the electroweak corrections, in both the \ms\
and the on-shell schemes of renormalization, are introduced, including the
decoupling of the top quark in certain amplitudes, its effect on
$\hat{e}^2(\mz)$ and \sincarmz, the incorporation of recent results on the
leading irreducible \Ord{\alpha^2} corrections, and simple expressions for the
residual, i.e.\ ``non-electromagnetic'', parts of \drcar, \drcarw, and \dr. The
results are used to obtain accurate values for \mw\ and \sincarmz, as functions
of \mt\ and \mh. The higher-order effects induce shifts in these parameters
comparable to the expected experimental accuracy, and they increase the
prediction for \mt\ derived from current measurements. The \ms\ and the
on-shell calculations of \dr, in a recently proposed formulation, are compared
and found to be in excellent agreement over the wide ranges $60\GeV \leq \mh
\leq 1 \TeV$, $\mz \leq \mt \leq 250 \GeV$.
