The distribution of small signal gain over 23 P- and R-branch lines in the 10.4 μm region of CO2 has been measured in a transversely excited high pressure CO2 laser system. The upper population density, the inversion density, and the temperature were determined by fitting a pressure-broadened gain distribution over the rotational–vibrational transitions at the time of maximum gain for pressures of 100, 400, and 755 Torr.