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Gain saturation of an Nd-glass amplifier by 1.06-μm light pulses is determined by the lifetimes and degeneracies of the two laser states as well as by thermalization rates among the components of the two multiplets to which the laser levels belong. Reported are measurements showing that although small signal gain was the same for amplified microsecond and picosecond pulses, comparable energy gain saturation occurred when the energy density of the mode-locked pulses was 1.9 times smaller than the amount for the microsecond pulses indicating terminal level filling with the shorter pulses. Our measurements indicate the terminal laser level, which is one of two unresolved groups in the4 multiplet, has a lifetime greater than 50 ns and a degeneracy likely to be one or two. Pulse train distortion due to saturating amplification was measured to find whether the thermalization rates among the levels of the laser multiplets were fast or slow compared to the 10-11-s pulses, but experimental uncertainties prohibit a determination.