The perturbation of a CW laser by a coupled optical transition is studied. In case of incoherent perturbation light, the laser photon number has been calculated recently by a fully quantum-mechanical approach. It is more easily obtained by simple rate equations, if the effect on the transition probability is considered by the introduction of a line broadening. In case of coherent perturbing radiation, a properly defined transition probability is derived from a semiclassical emission rate formula, given by FELD and JAVAN. The application on gas laser transitions reveals a dependence of the laser gain on the mutual directions of propagation of the laser light and the perturbation light.