  For a system at given temperature, with energy known as a function of a set
of variables, we obtain the thermal fluctuation of the evolution of the
variables by replacing the phase-space with a lattice and invoking the
principle of detailed balance. Besides its simplicity, the asset of this method
is that it enables us to obtain the Langevin equation when the phase-space is
anisotropic and when the system is described by means of curvilinear
coordinates. As an illustration, we apply our results to the
Kramer--Watts-Tobin equation in superconductivity. The choice between the
It\^{o} and the Stratonovich procedures is discussed.
