From consideration of the observational data on atmospheric nitric oxide, it is found that its concentration at any height must be less than 1011 cm−3. Theoretical deductions from an aeronomic study show that its concentration could be of the order of 108 cm−3 (or 5 × 108) at about 80 km, and 109 cm−3 (or 5 × 109) at about 65 km. At any height below 70 km the nitric oxide concentration must be less than the ozone concentration. The dissociation of nitrogen molecules involved in nitric oxide formation depends strongly on photoionization by ultraviolet or X-ray radiation and subsequent dissociative recombination. The vertical distribution of nitrogen atoms cannot be determined by a dissociation equilibrium, since it is subject to dynamical effects such as diffusion and atmospheric mixing. Whatever the processes involved in the formation of nitric oxide, its vertical distribution will tend to follow the atmospheric distribution until it becomes dissociated. A departure from photo-equilibrium conditions cannot occur for nitrogen peroxide molecules, for their lifetime in a sunldit atmosphere is very short. Nitrous oxide seems to play no role in the mesosphere. In anticipation of the importance attached to the aeronomic problem of nitrogen oxides in the mesosphere and lower thermosphere, various provisional numerical data are presented.