Choosing a simple oxygen-hydrogen model of the upper mesosphere, we study conditions necessary to create nonlinear effects such as cascades of period doubling and chaos. The model takes into account diurnal periodical excitation by solar radiation. The principal reaction of the model to water vapor changes and variations of the ratio of daytime hours to nighttime hours are studied in some detail. Although the model is rather simple as far as the considered photochemical processes are concerned, it is quite complex regarding the deterministic chaos because the phase space spans over five dimensions. Results of the calculations are discussed, including possible relations to measurements in the mesopause.¿ 1997 American Geophysical Union