  We show that in order to calculate correctly the {\it spin current} carried
by a quasiparticle in an electron liquid one must use an effective "spin mass"
$m_s$, that is larger than both the band mass, $m_b$, which determines the
charge current, and the quasiparticle effective mass $m^*$, which determines
the heat capacity. We present microscopic calculations of $m_s$ in a
paramagnetic electron liquid in three and two dimensions, showing that the mass
enhancement $m_s/m_b$ can be a very significant effect.
