π0-production in medium energy heavy ion reactions is studied via the Boltzmann-Uehling-Uhlenbeck approach. The mass dependence and bombarding energy dependence of the calculated absolute pion cross sections agree remarkably well with the recent data of Noll et al. for 12C(60, 75, 85 MeV/N) + C, Ni, Au. Assuming a reabsorption length of 3 fm, pion reabsorption occurs in >~ 50% of the events. Shadowing is important for the angular distributions of pions produced in heavy systems and also changes the apparent velocity of the ``pion emitting source''.Present address: Institut für Theoretische Physik der Universität Heidelberg, D 69 Heidelberg, Fed. Rep. Germany.