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We have analyzed changes in the acoustic oscillation eigenfrequenciesmeasured over the past 7 years by the GONG, MDI and LOWL instruments. Theobservations span the period from 1994 to 2001 that corresponds to half a solarcycle, from minimum to maximum solar activity.  These data were inverted to look for a signature of the activity cycle on thesolar stratification. A one-dimensional structure inversion was carried out tomap the temporal variation of the radial distribution of the sound speed at theboundary between the radiative and convective zones. Such variation couldindicate the presence of a toroidal magnetic field anchored in this region.  We found no systematic variation with time of the stratification at the baseof the convection zone. However we can set an upper limit to any fractionalchange of the sound speed at the level of $3 \times 10^{-5}$.Comment: 11 pages, 5 figures, to appear in Ap

An Upper Limit on the Temporal Variations of the Solar Interior Stratification