We have studied hyperbolic encounters of spherical galaxies by self-consistent N-body simulations. Each galaxy is represented by a Plummer model. A galaxy contains up to 250 “particles”. the force between the particles is properly softened in order to avoid an unrealistic internal evolution due to close encounters of the particles. the results of these numerical experiments, carried out by R.W., are compared with theoretical predictions based on the impulsive approximation, made by P.B. in order to facilitate the assessment of such a comparison, the passing galaxy is here represented by a rigid gravitational field, moving on a straight line. in the table given below, we compare the relative mass loss, ΔM/M, and the relative change in the total internal energy of a galaxy, ΔE/E, for two cases. Both galaxies are of equal mass and size; the impact parameter is equal to the median radius of a galaxy (containing 50% of its mass in projection); the initial relative velocity would correspond to the orbital excentricity e as given, if the galaxies were mass points. While the agreement is fair with respect to the energy transfer ΔE, the theory predicts a mass loss of more than 10% (or 25 particles) in cases where no mass loss is observed in the N-body simulations the probable reason for this discrepancy is the failure of the impulsive approximation in these cases. A mass loss of the order of 1% or less cannot be ruled out in the N-body calculations, because of the limited number of particles. the significant increase of the internal energy of a galaxy during an encounter leads to an expansion of the outer parts of the galaxy. This expansion may finally lead to a mass loss as soon as the outer parts have expanded beyond the tidal radius of the galaxy caused by its cluster environment. the inelasticity of galactic encounters has implications for the dynamical evolution of groups and clusters of galaxies. the groups and clusters will shrink in radius because of the loss of orbital energy which is used to expand the individual galaxies. the final amount of collapse of the group or cluster is governed by the relative amount of the internal binding energies stored initially in the individual galaxies and released during the galactic encounters, in comparison to the binding energy of the group or cluster.