The evolution of the pulse in an AM mode-locked TEA CO2laser has been investigated. The experiments have been performed by injecting the mode-locked pulses in a high-pressure slave oscillator at various time intervals after the initiation of the mode-lock process. This technique allows the measurements of the pulse widths independent on the pulse energies. A numerical solution of a dynamic model for the mode-locking process accurately predicts the transient evolution. It is shown that the build-up time to reach steady state can be, depending on the modulation depth, considerably larger than the duration of the pulse train.