  We discuss the transverse momentum distribution for the production of massive
lepton-pairs in hadron reactions at fixed target and collider energies within
the context of next-to-leading order perturbative quantum chromodynamics. For
values of the transverse momentum $Q_T$ greater than the pair mass $Q$, $Q_T >
Q$, we show that the differential cross section is dominated by subprocesses
initiated by incident gluons. Massive lepton-pair differential cross sections
are an advantageous source of constraints on the gluon density, free from the
experimental and theoretical complications of photon isolation that beset
studies of prompt photon production. We compare calculations with data and
provide predictions for the differential cross section as a function of $Q_T$
in proton-antiproton reactions at center-of-mass energies of 1.8 TeV, and in
proton-nucleon reactions at fixed target and LHC energies.
