Density modelling was carried out along five profiles oriented across the expected deep contact between the Bohemian Massif and the Internal Western Carpathians in western Slovakia. The density models reveal the continuation of the Bohemian Massif beneath the External and Internal Western Carpathians tectonic units. The eastern margin of the Bohemian Massif is situated at depth south-east of the surface outcrops of the Pieniny Klippen Belt and changes its position in the surveyed area. The contact of the Internal Western Carpathians with the Bohemian Massif and External Western Carpathians is subvertical. This sharp contact is manifested as the transtension to extension zone towards the surface.