Abstract : Regional seismic tomography can provide information on the deep structure of the old continents and therefore on their formation and evolution. We develop a tomography method using surface waves of teleseismic events at a regionalscale. This method is then applied to the Rayleigh waves recorded during the SVEKALAPKO deep seismic experiment in the central Baltic Shield. Our new method is based on two-dimensional ray tracing. It takes into account the curvature of incident wavefronts, therefore reducing artifacts due to heterogeneous structure outside the study region. From the inversion of Rayleigh wave data we obtain a regional average S-wave velocity profile with depth down to 300 km. This model is 4% faster than standard Earth models down to 200 km depth, it do es not exhibits any low velocity zone. A comparison with computations based on xenolith analysis leads us to propose a chemical stratification of the lithosphere. The shallower mantle is composed of a rock producing a seismic velocity slower than a normal continental lithospheric peridotite. SVEKALAPKO data are inverted to obtain a 3-dimensionnal S-wave velocity model under the seismic array down to 150 km. Lateral variations have amplitude of +/- 3% attributed to differences in the rock compositions. The major geological structure of the region - the Archean-Protcrozoic boundary - do es not present a clear signature in the velocity maps at different depths. However a classification of the velocity profiles with depth shows the juxtaposition of lithospheric blocs of different composition.