The generalized Langevin equation encapsulates in the friction kernel the influence of the environment on the reaction coordinate. We have decomposed the Langevin equation in a Fourier transform representation and analyzed how different Fourier components contribute to the friction kernel. We identified that a long-wavelength contribution to the friction kernel originates from collective solvent motions, and we compared it with contributions coming from short-range scattering of the complex with the surrounding solvent atoms. (C) 2003 American Institute of Physics.