A photochemical model of nitric oxide from 40 to 250 km has been used to study the distribution of NO in the mesosphere and lower thermosphere. The model includes a parameterization of NO photolysis that can account for the opacity of a varying thermospheric NO layer. This model is used to test the suggestion that increases in thermospheric NO which increases this opacity ''shield'' NO at lower altitudes by decreasing the photolytic destruction rate. The model was ''tuned'' to reproduce two recent observations of thermospheric NO, the high latitude rocket experiment of Eparvier and Barth <1992> and the midlatitude ground-based microwave experiment of Clancy et al. <1992>. In the first case, the model indicated that the large thermospheric NO layer could have caused significant shielding and up to a factor of 2 effect on the mesospheric NO is seen. On the other hand, for the midlatitude observation the NO optical pathlength was too small to be significant. A time dependent version of the model was used to study the response of mesospheric NO to auroal energy input during winter. Under conditions which combine rapid transport and NO self-shielding, the model suggests that NO density increases could propagate into the upper stratosphere. ¿ American Geophysical Union 1994