The degassing history of 40Ar on Mars is reconstructed from a model based on the Martian volcanic record. Accumulation history of 40Ar is influenced by the volcanic eruption rate at each Martian epoch in addition to the production rate of 40Ar due to potassium decay in the mantle. In order to degas the present amount of 40Ar to the Martian atmosphere, the volume fraction of erupted magma to the original mantle materials averaged over history should be about 0.006--0.018. The integrated volume of mantle materials that produced the observed volcanic materials and atmospheric 40Ar is about (4--11)¿1018 m3, hence the extent of mantle differentiation after the formation of Mars is only 0.03--0.08. A significant amount of 40Ar (about 73% of the present atmospheric content) has apparently been degassed during the formation of the major volcanic regions on Mars, such as Tharsis and Elysium. This means that a significant portion of 40Ar in the Martian atmosphere would have been degassed by relatively recent volcanic activity, whereas large fractions of other volatile components, such as H2O and CO2, might have been degassed by an early catastrophic degassing. The possibility of ancient plate tectonic activity on Mars, which was proposed recently to explain the origin of the Martian crustal dichotomy, is also examined by using the 40Ar degassing model. Although the possibility of Martian plate tectonics may not be ruled out from the model, we can constrain an upper limit for the duration of plate tectonics. The duration should be much shorter than 6--350 Ma (probably ≪100 Ma), which is rather short compared with the ranges of the northern lowlands ages (600--750 Ma). ¿ American Geophysical Union 1996