We have developed a modelling system for predicting the emissions, dispersion and chemical transformation of nitrogen oxides in an urban area. The system takes into account of all source categories, including stationary point and area sources, and vehicular sources. This paper describes a comparison of the predicted NO% and NOi concentrations with the results of an urban air quality measurement network. The comparison included four monitoring stations in the Helsinki metropolitan area in 1993. The predicted and measured NOi concentrations agreed well at all the stations considered. The agreement of model predictions and measurements for NOx and NC>2 was better for the two suburban measurement stations, compared with the two stations located in downtown Helsinki