A generalization of the generalized Langevin equation (stochastic dynamics) is introduced in order to model chemical reactions which take place in changing environments. The friction kernel representing the solvent response is given a non-stationary form with respect to which the instantaneous random solvent force satisfies a natural generalization of the fluctuation-dissipation relation. Theoretical considerations, as well as numerical simulations, show that the dynamics of this construction satisfy the equipartition theorem beyond its equilibrium limits.