Possible photochemical schemes resulting in the formation of nitric oxide i
n the mesosphere are considered. The results of numerical calculations of t
he [NO] altitude distribution, which satisfactorily agree with experimental
data, are presented. It has been shown that the [NO] can be in a photochem
ical equilibrium in the mesopause. The analytical expressions for the [N] a
nd [NO] altitude distribution in the mesosphere have been obtained.