Experimental and modeling results on CO laser frequency conversion are presented. A Q-switched multiline CO laser with pulse repetition rate 20-150 Hz of sub-microsecond pulses and electron beam sustained discharge frequency-tunable mode-locked CO laser were used in the experiments on second harmonic generation (SHG) in high-quality ZnGeP2 and GaSe crystals. Internal SHG efficiency exceeded 12.4 % in 12 mm ZnGeP2 crystal. The SHG in 4 mm GaSe crystal was observed with internal efficiency of 0.3%. A possibility of difference frequency generation of fundamental and firstovertone CO laser lines to cover spectral range of ~4.0-5.0 μm is discussed. It is estimated that the difference frequency generation of neighboring lines of both fundamental and first-overtone bands allows one to obtain oscillation in THz spectral range within ~200-3000 μm.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.