The curves of anomalous dispersion of a minor impurity of a molecular gas in a buffer gas are calculated taking into account laser–induced distorsions of Maxwell's molecular velocity distribution at the levels resonant with laser radiation. It is shown that account of this nonequilibrium may lead to an increase in the absolute values of variations in the refractive index by several times. In this case the result depends strongly on the ratio of the velocities of elastic (T–T) and rotational (R–T) relaxations of molecules. In the limiting case of slow elastic relaxation a simple formula for the refractive index is obtained and its range of applicability is analyzed. A parameter of resonance self–focusing of the CO2–laser radiation in the atmosphere is estimated taking the laser–induced distortion of the Maxwellian distributions into account.