Excitation of the A(3s sigma) Rydberg state of NO leads to an extensive rearrangement of the environment, which we have investigated by classical molecular dynamics simulations and normal mode analysis, using pair potentials from the literature. We find that the medium response is independent of the details at long range of the excited state NO A-Ar potential, stressing the fact that it is mainly driven by the short range repulsive forces between the Rydberg electron and the matrix atoms. We establish the inertial character of the first shell response in the initial 100-150 fs after excitation, as the next shells are silent over this time scale. The expansion of the first shell at early times, induces the propagation of a supersonic wave along the (011) axis of the crystal, which define 12 linear chains of atoms with the impurity. The early time response is followed by vibrational coherences with a complex behavior. The normal modes analysis of the crystal shell by shell shows good agreement with the power spectra of the MD trajectories. It allows us to identify the most significant modes in the medium response. Overall, the dynamics of the system may be regarded as that of a NOAr12 supermolecule, embedded in an Ar lattice and undergoing vibrational energy redistribution. (C) 2001 American Institute of Physics.