  Spin-polarized symmetric nuclear matter is studied within the
Dirac-Brueckner-Hartree-Fock approach. We pay particular attention to the
difference between the one-body potentials of upward and downward polarized
nucleons. This is formally analogous to the "Lane potential" for
isospin-asymmetric nuclear matter. We point out the necessity for additional
information on this fundamentally important quantity and suggest ways to
constrain it.
