The reaction of adiabatic electron transfer in a polar solvent with two relaxation periods is studied. The dynamics of the process is described by a stochastic non-Markovian equation with memory. This equation reduces to a system of two Markovian equations. Calculations of the rate constant within the framework of generalized transition-state theory (equivalent to the Grote-Hynes theory) are not satisfactory if the relaxation periods differ greatly. The non-equilibrium rate constant is obtained by reducing the appropriate Fokker-Planck equation to the Sumi-Marcus equation. Various kinetic regimes are examined for the latter equation, depending on the ratio of relaxation periods. Rate constants are estimated for the reactions of outer-sphere electron transfer in aliphatic alcohols.