It has generally been supposed that the rise and fall times of the gain in pulsed CO2laser discharges reflect the effective lifetimes of the lower and upper laser levels. Consequently, the ratio of the upper to lower level populations should change during the evolution of the gain, and should be a relatively large number at the gain peak. The problems associated with the determination of this ratio using conventional direct gain measurements are discussed, and a new in-cavity technique is described that has sufficient precision to determine the ratio as a function of time. The results obtained unambiguously demonstrate that the lower laser level empties rapidly during the rise time of the gain. This conclusion is further reinforced by some measurements concerning the time variation of the gain in the hot band ( ).