  We derive a generalized form of Altarelli-Parisi equations to decribe the
time evolution of parton distributions in a nuclear medium. In the framework of
the leading logarithmic approximation, we obtain a set of coupled integro-
differential equations for the parton distribution functions and equations for
the virtuality (``age'') distribution of partons. In addition to parton
branching processes, we take into account fusion and scattering processes that
are specific to QCD in medium. Detailed balance between gain and loss terms in
the resulting evolution equations correctly accounts for both real and virtual
contributions which yields a natural cancellation of infrared divergences.
