  We study the evolution of an oscillator interacting via the most general
bilinear coupling (with time-independent coefficients) with an ``environment''
consisting of a set of other harmonic oscillators. We are mainly interested in
a possibility of using the Fokker-Planck equation to describe this evolution.
Studying different interaction Hamiltonians, we show that unambiguous reduction
to the Fokker-Planck equation is possible only within the framework of the so
called rotating-wave approximation. As special cases we consider in detail the
evolution of two coupled oscillators and relaxation of a charged oscillator in
a uniform magnetic field.
