  We introduce a distinctive feature of spin-polarized transport, the Spin
Coulomb Drag: there is an intrinsic source of friction for spin currents due to
the Coulomb interaction between spin ``up'' and spin ``down'' electrons. We
calculate the associated ``spin transrestistivity'' in a generalized random
phase approximation and discuss its dependence on temperature, frequency, and
electron density. We show that, in an appropriate range of parameters, such
resistivity is measurable and propose an experiment to measure it.
