Relaxation processes in polymethylmethacrylate were investigated using light scattering and mechanical relaxation techniques. The acoustic attenuation data cover the frequency range from 1 Hz to 17 GHz and the light scattering from 0.5 to 10 000 GHz. Analysis of the data in a broad temperature range (80-350 K) shows that the model of the asymmetric double-well potentials, traditionally used for description of the acoustic attenuation in glasses, cannot describe all the data consistently. It is shown that assuming an additional relaxation in GHz region with a constant loss spectrum provides a good agreement of the model with experimental data.