A review is given of the investigations of the mechanism of population inversion of vibrational levels in gas-discharge CO lasers operating at room and liquid nitrogen temperatures. Special attention is paid to the published theoretical and experimental information on the probabilities of vibrational transitions in molecules. Calculations of the populations of vibrational levels in the CO laser are discussed in detail. An analysis of these investigations demonstrates the correctness of the hypothesis postulating that the vibrational degrees of freedom of the CO molecule are pumped by electron impact and the higher levels are populated by the relaxation of carbon monoxide, which can be regarded as the relaxation of a system of anharmonic oscillators as first suggested by Treanor.