The distribution function for two circularly polarized modes in a laser with axial magnetic field is calculated. If the resonator has different damping constants for waves polarized in different directions, we find a coupling between the modes. To describe this coupling it is necessary to couple both modes to the same heatbath. Starting from a masterequation describing the damping of the atoms and the light field, we may transform the masterequation to a Fokker-Planck equation by introducing a quasi distribution function. By eliminating the atomic variables adiabatically we find a Fokker-Planck equation for the light field variables only. The solution of the Fokker-Planck equation is given in the stationary case above threshold and in the non stationary case below threshold.