The feasibility of an adiabatic treatment of beta vibrations for even-even rare-earth nuclei is investigated with the restricted formula of Davydov. The parameters gamma and mu as deduced from the fitting of experimental data with Davydov-Chaban predictions are used as zeroth-order approximation. For all the considered nuclei, at a certain stage of the iterative process, the parameters got out of the range of validity of the restricted formula. Taking into account the values of those parameters for other regions of the periodic table, the conclusion can be drawn that an adiabatic treatment of beta vibrations within the frame of the Davydov model cannot be made.