The behavior of nitrogen oxides in the stratosphere and mesosphere is discussed with the aid of a model which introduces the photodissociation of nitric oxide and the formation of nitric acid. The profiles of the nitric oxide, nitrogen dioxide and nitric acid concentrations are sensitive to the values of the eddy diffusion coefficients which are adopted. The evaluation of the various reactions which enter the stratosphere shows the role of the formation of nitric acid which is related to the production of OH radicals in the lower stratosphere. An increase of the water vapor in the stratosphere leads to a decrease of nitric oxide and nitrogen dioxide.