The balance between the production and loss rates of CO in the Martian atmosphere is examined in the photochemical frame defined by Parkinson and Hunten (1972). A first important point is that, for any given value of the CO abundance, the balance of CO is roughly reached in a short lapse of time through balances of short lived O, O3 and OH. From this basic fact, we suggest a quite interesting characteristic of the photochemistry of carbon compounds on Mars, implicit in earlier models, but not yet discussed in detail. Because the production of OH is the rate-limiting step in oxidation of CO, there is in first approximation no feedback process regulating CO at any particular level. A more refined analysis shows that feedbacks exist, although there are weak. Damping processes are identified and their efficiency estimated in a rather general frame. A corollary is the intrinsic instability of the equilibrium abundance of CO, which is very sensitive to fluctuations of its production and loss rates. Nevertheless, because of the long life time of CO, temporal variations (seasonal, solar cycles) are expected to be weak (≈¿10%). Large latitudinal gradients predicted in static conditions are likely smoothed by dynamical mixing. The contribution of the present ideas to the understanding of the recent Earth-based and space observations of the Martian CO is finally evaluated. Some new directions of research are proposed. ¿American Geophysical Union 1991