A determination of the mixing ratio of 14CO2 in the Martian atmosphere would provide a direct measurement of the degree of exchange of CO2 in the Martian atmosphere with CO2 in the regolith and polar caps. This is the case because production of 14C occurs in the atmosphere and in the very-near-surface regolith and decays away in a timescale of 104 years. The resulting steady state abundance in the atmosphere can be diluted by mixing with nonatmospheric reservoirs. The degree of mixing expected is such that an order-of-magnitude measurement would provide definitive information on the state of the regolith. Although in situ measurements probably are not possible, determination could be made easily on a sample of pristine Martian atmosphere returned to the Earth. ¿ American Geophysical Union 1996