Recent measurements of water composition in SNC meteorites have shown a high and variable deuterium to hydrogen (D/H) ratio. The highest D/H value is nearly equal to the present-day value in the Mars atmosphere. This indicates that between 1.3 billion years (Gyr) ago and 180 million years (Myr) ago the Martian atmosphere may have attained the same D/H ratio as it has now. This also provides evidence for an interaction process involving water operating between atmosphere, regolith, and rocks in SNC meteorites. In this paper we have calculated the amount of water at present and at 4.5 Ga via the D/H fractionation process, assuming that water is lost to space as well as to the surface. A surface flux smaller than ~108 cm-2 s-1 operating over the life history of the planet will not have any influence on the size of the fractionated water reservoir. However, we find that substantial surface loss of water will be required in case Mars' exchangeable water inventory has undergone transition from an initial large abundance to a small amount at present. Our results show that water which is lost to the surface would carry the D/H signature of the atmosphere of that period. ¿ 1998 American Geophysical Union