A conceptual model for simultaneous computation of runoff and inorganic nitrogen leaching is described. The model is semi-empirical with coefficients that are calibrated against observed data. It has been developed and tested for mixed basins consisting of forest, arable land and lakes. In two of the basins, lakes have a major impact on the transport of nitrate. The results show that a major part of the variations of inorganic nitrogen in runoff can be explained by the model, but details in the modelling of nitrogen turnover can be masked by biological and physical processes in the lakes.