  The lepton angular distributions of the Drell-Yan process in fixed-target
experiments are investigated by NLO and NNLO perturbative QCD. We present the
calculated angular parameters $\lambda$, $\mu$, $\nu$ and the degree of
violation of the Lam-Tung relation, $1-\lambda-2\nu$, for the NA10, E615 and
E866 experiments. Predictions for the ongoing COMPASS and SeaQuest experiments
are also presented. The transverse momentum ($q_T$) distributions of $\lambda$
and $\nu$ show a clear dependence on the dimuon mass ($Q$) while those of $\mu$
have a strong rapidity ($x_F$) dependence. Furthermore, $\lambda$ and $\nu$ are
found to scale with $q_T/Q$. These salient features could be qualitatively
understood by a geometric approach where the lepton angular distribution
parameters are expressed in terms of the polar and azimuthal angles of the
"natural axis" in the dilepton rest frame.
