The objective of this project is to calculate national and regional nitrogen (N) balances and study the effect of nitrogen balance on nitrogen leaching using dynamic simulation models. National nitrogen balance has decreased from 90 kg ha -1 in 1990 during the environmental programme of 1995-2002 to 50 kg ha-1. The decrease in regional nitrogen balances that are calculated by rural centres is for same magnitude. The COUP model is used to calculate field scale N balances for five farms of different production system. The COUP model was calibrated to measured inorganic nitrogen amounts in autumn 2000. The inorganic N leached between January and Sebtember from field fertilised by cattle slurry was near the average leaching measured in Finnish fields. Inorganic N leaching from three field parcels was low and vegetation used inorganic N effectively. In two parcels yield was low, so vegetation did not take nitrogen effectively, and N leaching was high. A model system based on ICECREAM model has been developed to be used for assessing potential nitrogen leaching in agricultural districts. Input data consisted of information about typical crops and soils. The modelling system was tested first in Southern Finland with data from Häme and Farma agricultural districts. Annual potential nitrogen leaching was calculated for the period 1990-1999 in a scenario, where agricultural practices were same for the both regions. This base line scenario showed that climatic and soil conditions have a clear impact on the leaching. The representativity of the available climatic data (location of the stations in different agricultural districts) was analysed by GIS. In the first phase of the modelling study leaching of nitrate N was calculated using data from six agricultural districts. Spatial and annual leaching varied highly for different years with a standard fertilising amount 90 kg/ha/a. When the model exercise was repeated with a decreasing fertiliser amount (110 kg/ha/a => 90 kg/ha/a), a clear decrease in nitrate leaching was observed.