  Open charm production at the initial and the pre-equilibrium stages in
nuclear collisions is studied. The pre-equilibrium contribution to open charm
production in $A+A$ collisions at $\sqrt s=200 AGeV$ is shown to be very
sensitive to correlations between momenta and space-time coordinates of the
minijets produced after the initial collision. A minimal correlation model
based on the uncertainty principle is introduced, and leads only to a small
pre-equilibrium charm yield as compared with initial yields. Opposite-sign
lepton pairs $ee, e\mu$ and $\mu\mu$ from open charm decay in $p+A$ reactions
at $\sqrt s=200$ AGeV are proposed as a measure of nuclear shadowing effects.
Via an approximate scaling, the ratio of the spectra from $p+A$ collisions to
those from $pp$ collisions is shown to reflect the shadowing function well.
Finally it is shown that the required measurements are feasible at RHIC.
