The effect of the modulation of the electronic wave functions by configurational fluctuations of the molecular environment on the kinetic parameters of electron transfer reactions is discussed. A self-consistent algorithm for the calculation of the potential profile along the reaction coordinate of adiabatic electron transfer reactions is elaborated. A new formula for the transition probability of non-adiabatic electron transfer reactions is obtained in an improved Condon approximation A regular method for the calculation of non-Condon corrections is suggested. The importance of these effects for some specific biological and electrochemical electron transfer systems is discussed.