We present an analysis of the location of interacting E/S0 galaxies on the Fundamental Plane (FP). Using the NEMO package, we performed N-body simulations of close encounters and mergers of two self-gravitating spherical galaxies. Two models for encounting galaxies - Plummer's and Hernquist's - were used. The number of particles ranged from 20 000 to 50 000 per galaxy. The changes of central density, half-mass radius and central velocity dispersion were analysed. It was found that close encounters between galaxies alter noticeably the above parameters within a very short time interval (10(7)-10(8) years) just before the final merger. The amplitudes of parameter changes strongly depend on the initial mass concentration of a model. In some experiments we considered an encounter of two spherical galaxies with dark matter components. The effect of parameter changes was less pronounced than for the experiments without a dark halo. The results of the simulations were used to discuss the FP for interacting early-type galaxies.