  We study the pion production in heavy-ion collisions at SIS energies within
the coupled resonance BUU model. The standard calculation overpredicts the pion
multiplicity in the central Au+Au collision at 1 A GeV by about a factor of 2,
which is interpreted as a signature for the quenching of the resonance
production/absorption in nucleon-nucleon collisions at high nuclear density.
Results of the calculations taking into account a specific form of quenching
are presented for pion spectra from light and heavy systems as well as for the
pion in-plane and out-of-plane flow.
