A calculation of the amount of volatiles that Mars has lost in the past through solar wind erosion is presented. The analysis is based on the examination of the ionospheric plasma flow produced by the transfer of momentum of the solar wind to the planet's upper ionosphere and is carried out by using loss rates suitable to a dense Venus-like Martian early ionosphere. The results indicate that an amount of mass equivalent to that of a global water ocean at least 10 m deep was removed by this process over the planet's lifetime. Further calculations show that depending on the strength of the Martian early ionosphere the total mass eroded could have been the equivalent of a global ocean up to ~30 m deep. These numbers are much larger than those derived from current planetary escape rates inferred from the recent Phobos measurements and represent up to about nearly one half of the amount of water which is believed was delivered to the Martian atmosphere through early volcanic activity. ¿ American Geophysical Union 1992