  We discuss dilepton production from a quark-gluon plasma which has a
time-dependent anisotropy in momentum space. A phenomenological model for the
hard momentum scale, p_hard(tau) and the plasma anisotropy parameter, xi(tau),
is presented. The model interpolates between early-time 1+1 free streaming
behavior (tau << tau_iso) and late-time ideal 1+1 hydrodynamical behavior (tau
>> tau_iso). Using this model, we find that the dilepton rate arising from
electromagnetic annihilations of quarks in the kinematic range 3 <p_T< 8 GeV is
sensitive to the assumed isotropization time of the system, tau_iso. Therefore
high-energy dilepton production can be used to probe the degree of
momentum-space isotropy of a quark-gluon plasma produced in relativistic heavy
ion collisions and the time of onset of hydrodynamic expansion of the QGP.
