For the grounding of the program of the wide-aperture several THz bandwidth CO2 laser amplifier creation it was initiated the experimental investigation of a HF laser radiation transformation in a multiatmospheric C02/He medium. The C02/3He mixture (46x50x63 mm3, 4 atm) was pumped by the 1P8 spectral line of the pulsed chemical HF laser (85 J, 1 ps). Transverse pumping geometry was used. Up to the power density of 2.7 MW/cm2 an absorption saturation was not observed. At the deposited energy of 58 J and output coupling of 20% it was obtained CO2 laser pulse of 2.7 J (quantum efficiency was about 1 8.6%). The estimated small-signal gain was of 0.05 cm1