  We present the full next-to-leading order (NLO) corrected inclusive cross
section for massive lepton pair production in longitudinally polarized
proton-proton collisions. All QCD partonic subprocesses have been included
provided the lepton pair is created by a virtual photon, which is a valid
approximation for $Q<50 {\rm GeV}$. Like in unpolarized proton-proton
scattering the dominant subprocess is given by the $q(\bar q)g$-channel so that
massive lepton pair production provides us with an excellent method to measure
the spin density of the gluon. Using our calculations we give predictions for
the longitudinal spin asymmetry measurements at the RHIC.
