We investigate the combined electronic and nuclear motion in a simple model system as proposed by Shin and Metiu [J. Chem. Phys. 102, 9285 (1995)]. A variation of the electron-nuclei interaction energy allows us to study the transition from an adiabatically uncoupled situation to the case where the adiabatic approximation breaks down. Wave-packet calculations illustrate the dynamical changes of electronic and nuclear probability densities for the case of an adiabatic motion and during a nonadiabatic transition. The influence of the coupling on the transport of electron density between different nuclei is characterized with the help of time-dependent Laplacians. (C) 2003 American Institute of Physics.