There has been a big advance in the knowledge of the composition of the atmosphere of the planet Mars since its exploration by different missions in the 1970s, and this will be deeply increased in the following years as the upcoming programs to Mars develop. In this context, we have elaborated a model of the Mars' neutral atmosphere including the following compounds: O(3P), O(1D), O2, O3, H, H2, OH, H2O, HO2, H2O2, CO, and CO2, between 30 and 200 km of altitude. The model is carried out for middle latitudes in equinox conditions and with moderate solar activity and provides the day-to-night evolution of the atmosphere. The scarcity of observations corresponding to the nightside of the planet has made it necessary to calculate the atmospheric temperature profile based on the available observations and on theoretical estimations. the model includes a detailed treatment of both the photochemical and the dynamical processes. In this sense, the most recent values of the reaction rates and photodissociation cross sections have been used, and a new height profile of the eddy diffusion coefficient has been computed which is able to explain the vertical distribution of carbon monoxide. The concentration profiles obtained show, in general, a very good agreement with the available experimental measurements. ¿American Geophysical Union 1990