The classical molecular dynamics simulation method was applied to the study of the electron transfer (ET) occurring in the optically excited methylene blue-guanine complex in water. The basic assumptions of Marcus model for ET in polar solvents were proven to be valid in the present case: the solvent orientation in the solute electric field was found to be the actual reaction coordinate, and the free energy dependence on reaction coordinate was found to be parabolic. The electronic contribution to the energetics of the process was calculated as the difference between the ionization potentials of reducing and that or the reduced species. On the basis of the free energy curves computed for the locally excited state (LE) and the charge transfer state (CT) a reaction activation energy of 2.0 kcal/mol and a reorganization energy of 27.1 kcal/mol were derived. Solvent and solute dynamics were analyzed by computing the time auto-correlation functions for the corresponding energy terms. Both solvent and internal relaxation were found to be nonexponential but with different time constants: 20 fs, 400 fs, and 1.5 ps for solvent and 40 fs, 1 ps, and 5 ps for internal relaxation. It was also found by analyzing simulated reaction trajectories that the transition state approximation is valid for the present electron transfer process if the coupling between the LE. and the CT state does not significantly modifies the free energy barrier.