We study transport in ferromagnetic single-electron transistors. The non-eq
uilibrium spin accumulation on the island caused by a finite current throug
h the system is described by a generalized theory of the Coulomb blockade.
It enhances the tunnel magnetoresistance and has a drastic effect on the ti
me-dependent transport properties. A transient decay of the spin accumulati
on 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.