In the framework of the EU-project RANR (Regional analysis of subsurface nitrogen retention and its impact on the nitrogen export from land to sea) a model system was developed to estimate groundwater-borne nitrogen inputs into river systems. The core of this model system is composed of a soil nitrogen leaching model (SOIL-N) and a groundwater residence time/denitrification model (WEKU). The application of the model system was carried out for the study catchment areas of the Uecker basin (ca. 2400 km(2), Germany) and the Gjern basin (ca. 200 km(2), Denmark). For both catchment areas, the modelled average nitrogen loads leached into the groundwater were about 40 kg N/ha a, while the remaining groundwater-borne nitrogen intake to rivers was quantified to an average of about 2 kg/ha a. The comparision with observed groundwater-borne riverine nitrogen loads showed a very good agreement, proofing the key role nitrogen retention in groundwater plays in the two catchment areas. With regard to the generalisation and transfer of the SOIL-N/WEKU model concept we assume that the model can be applicated in catchment areas in the European Pleistocene Lowland, which ranges from the Netherlands in the west to the Baltic States and the Ukraine in the east.