Abstract. We present the essential dynmnic model of the •nesospheric photochemical system (PCS) •'md suggest a step-by-step procedure for elaborating such a model of an arbitrary atmospheric PCS. The model demonstrates the same possibilities of nonlinear dynamic behavior and qualitatively the same dynamic characteristics as the corresponding original model, but is much simpler than the latter. We show the adequacy of the essential model compared with the original one in bifurcation diagrams, equilibrimn states, and such new characteristics as correlation dimension and minirotan embedding dimension of a chaotic attractor. The model can be used both for identi•ing and studying the mechanisms of the nonlinear dynmnic behavior of the mesaspheric PCS, as well as for solving a number of problems aimed at revealing nonlinear photochemical phenomena in the actual mesasphere.