Abstract: A method and apparatus is provided for stabilizing output beam parameters of a gas discharge laser by maintaining a constituent gas of the laser gas mixture at a predetermined partial pressure using a gas supply unit and a processor. The constituent gas of the laser gas mixture is provided at an initial partial pressure and the constituent gas is subject to depletion within the laser discharge chamber. A parameter such as time, pulse count, driving voltage for maintaining a constant laser beam output energy, pulse shape, pulse duration, pulse stability, beam profile, bandwidth of the laser beam, temporal or spatial coherence, discharge width, or a combination thereof, which varies with a known correspondence to the partial pressure of the constituent gas is monitored. Injections of the constituent gas are performed each to increase the partial pressure by a selected amount in the discharge chamber.