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Gain saturation in the CO2 laser system presents a limitation on the amplification of Q-switched pulses and this has been studied in some detail. With pulses of around 0.5 Â¿s saturation effects are observed at power densities of order 104W cmÂ¿2. The results of a theory of the time dependence of saturation are presented, together with supporting experimental data. This theory indicates that, at high power levels, the decrease in gain with time during the oscillator pulse can be described by the sum of two exponential decays. A value for the relaxation rate of the rotational sub-levels of the active vibrational states is obtained for various gas conditions. The implications of this relaxation rate on the design of high power oscillator/amplifier systems is discussed.