  The anisotropy in the temperature dependence of the in-plane and c-axis
conductivities of high-T_c cuprates in the superconducting state is shown to be
consistent with a strong in-plane momentum dependence of both the quasiparticle
scattering rate and the interlayer hopping integral. Applying the cold spot
scattering model recently proposed by Ioffe and Millis (Phys. Rev. B {\bf 58},
11631 (1998)) to the superconducting state, we find that the c-axis
conductivity varies approximately as T^3 in an intermediate temperature regime,
in good agreement with the experimental result for optimally doped
YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}.
