Calculations are compared with measurements of nanosecond pulses propagating through a large CO2‐laser amplifier system. Both coherence of medium polarization and rotational relaxation are included in the traveling‐wave equations describing pulse propagation in an amplifying or absorbing medium. The basic formalism is derived and adapted to a model of the LASL two‐beam system. Excellent agreement between the calculated and the measured output pulse shape and energy extraction is found. No adjustable parameters influence these calculations since all physical parameters used, as well as the input pulse itself, are derived from measured quantities.