ABSTRACT: The most important role in groundwater accumulation in the Bytčianska kotlina and Ilavská kotlina basins is played by Quaternary fluvial sediments of the river Váh and proluvial sediments of smaller lateral valleys on the margins of both basins. Aquifer hydraulic properties in the Bytčianska kotlina basin were characterized using data on 92 boreholes drilled in the fluvial sediments of the Váh river and 3 wells in the fluvial terrace sediments of the Váh river. In the Ilavská kotlina basin, 100 boreholes from fluvial sediments of the Váh river flood plain and 26 fluvial sediments of the Váh river Pleistocene terraces. Proluvial sediments on the edges of both basins were characterized on 5 boreholes. Groundwater regime was evaluated on the basis of observations on 7 monitoring wells provided by SHMI (Slovak Hydrometeorological Institute) in the Bytčianska kotlina basin and 14 in the Ilavská kotlina basin for the period of 1993 - 2002. Regime of Quaternary aquifers of both basins was characterized using annual average position of groundwater table and also representative minimum and maximum position of level were chosen. Regularized spline method with tension was used for the interpolation of groundwater levels in the basins.