  A study of the angular distributions of leptons from decays of J/psi's
produced in p-C and p-W collisions at sqrt{s}=41.6 GeV has been performed in
the Feynman-x region -0.34 < x_F < 0.14 and for transverse momentum up to 5.4
GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring
of the DESY laboratory. The results, based on a clean selection of 2.3 x 10^5
J/psi's reconstructed in both the e^+ e^- and mu^+ mu^- decay channels,
indicate that J/psi's are produced with longitudinal polarization. The
magnitude of the effect is maximal at low p_T. For p_T >1 GeV/c a significant
dependence on the reference frame is found: the polar anisotropy is more
pronounced in the Collins-Soper frame and almost vanishes in the helicity
frame, where, instead, a significant azimuthal anisotropy arises.
