Lake Vostok is the largest subglacial lake in Antarctica. We show that as a result of its size tidal and atmospheric pressure forcings are large enough to induce vertical displacements of the ice surface above the lake. These effects have been modelled assuming equilibrium tides and an inverse barometer response based on mass conservation. Differences in the tidal potential on the area of the lake result in height changes with amplitudes of a few millimetres for the largest diurnal and semidiurnal tides with maximum values at the southern end of the lake. Air pressure differences above the lake cause a differential inverse barometer effect (IBE) with resulting height changes of up to 40 mm on timescales from days to weeks. Both effects could be verified by differential Global Positioning System (GPS) observations in the southern part of the lake during the 2001/2002 and 2002/2003 summer seasons. The measured amplitudes and phases of the main constituents correspond to the respective values of the differential equilibrium tides. ERS-1& 2 tandem interferograms of 1996 were used to analyse the spatial pattern of the surface displacement. For the southern part of the lake, these measurements reveal a bulge with a wide flexure zone of varying amplitude but a similar geometry. A quasi-simultaneous pair of interferograms from the northern and the southern part of the lake respectively, demonstrates surface height changes of opposite sign in both areas. Therefore, it can be concluded that the associated water redistribution as a result of tides and changing atmospheric pressure forms an additional component of the overall water circulation in the lake.