We present analytical results for the leading contributions of the top
quark to the electroweak rho parameter at order GF^3 Mt^6 and
GF^2 Mt^4 alpha_s. The Higgs boson and the gauge bosons are taken
to be massless in this limit. The correction of order GF^3 Mt^6
is found to be sizeable in comparison to the the leading two-loop
GF^2 Mt^4 correction, however it is much smaller than
the subleading GF^2 Mt^2 MZ^2 correction.