  The large value of the top quark mass implies that the rare top decays $t
\rightarrow b W^+ Z, s W^+ Z$ and $d W^+ Z$, and $t \rightarrow c W^+ W^-$ and
$u W^+ W^-$, are kinematically allowed decays so long as $m_t \ge m_W + m_Z +
m_{d_i} \approx 171.5 GeV + m_{d_i}$ or $m_t \ge 2m_W + m_{u,c} \approx 160.6
GeV + m_{u,c}$, respectively. The partial decay widths for these decay modes
are calculated in the standard model. The partial widths depend sensitively on
the precise value of the top quark mass. The branching ratio for $t\rightarrow
b W^+ Z$ is as much as $2 \times 10^{-5}$ for $m_t = 200 GeV$, and could be
observable at LHC. The rare decay modes $t \rightarrow c W^+ W^-$ and $u W^+
W^-$ are highly GIM-suppressed, and thus provide a means for testing the GIM
mechanism for three generations of quarks in the u, c, t sector.
