A nitrogen budget is presented for the Belgian watershed. Such a budget, established at the scale of a hydrographical network, requires to quantify the magnitude of the nitrogen inputs, transformations and transfers which occur in this network. Nitrogen inputs in the hydrographical system are related essentially to soil drainage and human activities. Nitrogen soil drainage has been measured by use of the small watershed method. Domestic load has been evaluated from population census. Industrial effluents have been quantified by use of the evaluation of specific nitrogen spoilage by the various industries as a function of their number of workers. A fraction of the cattle wastes production in not spread on agricultural soils and rejoins the rivers. An estimation of this fraction is presented for the Belgian watershed. Denitrification, at the scale of a whole hydrographical network, is difficult to estimate accurately. A semi-empirical model of the dependence of benthic denitirifcation on the organic matter content of the sediments has allowed evaluation of its rate at the scale of the Belgian hydrographical network. This paper stresses the predominant role of denitrification in the Belgian hydrographical network as a sink which reduces the amount of nitrogen exported to the North Sea or the adjacent basins. It is therefore likely that any modification of the factors affecting denitrification in the hydrographical network could deeply affect the inputs of nitrogen into the coastal areas. Application of the denitrification-model has suggested that the pursuit of the present waste water treatment policy, only based on the elimination of the organic load without any tertiary treatment, would result in a improvement of the quality of the river system, but could paradoxically result in an raise of nitrogen discharge into the coastal waters, increasing the risks of eutrophication. ; SCOPUS: NotDefined.j ; info:eu-repo/semantics/published