  We study transport in ferromagnetic single-electron transistors. The non-
equilibrium spin accumulation on the island caused by a finite current through
the system is described by a generalized theory of the Coulomb blockade. It
enhances the tunnel magnetoresistance and has a drastic effect on the time-
dependent transport properties. A transient decay of the spin accumulation may
reverse the electric current on time scales of the order of the spin-flip
relaxation time. This can be used as an experimental signature of the non-
equilibrium spin accumulation.
