  The validity of the Langevin equation (both classical and quantum) is studied
in cases when not all the equations of motion are linear. In particular, a
model is studied in which the interaction is bilinear in the environment
variables. We conclude that the equation is valid only for frequencies close to
the frequency of the system alone. If the equation of motion of the system
alone is nonlinear, we are unable to find a condition for the validity of the
Langevin equation.
