Abstract This study describes an application of the APRAC3 (Air Pollution Research Advisory Committee Version 3) model of the EPA UNAMAP (User's Network for Applied Modeling of Air Pollution) system to the calculation of the CO and NO x air concentrations from motor vehicle emissions in the Milan urban area. The local micrometeorological parameters were measured by an automatic monitoring network; traffic flows and speed were evaluated through a traffic assignment model; motor vehicle composition and hourly distribution throughout the day were determined by the Milan Urban Police measurements; the emission factors were estimated from R. Joumard's experimental data. The calculated hourly concentrations of CO and NO x for the average day were compared with those measured at six points fortwo months, June and September 1989. The normalized mean-square errors obtained for the monthly average hourly concentrations were generally below 0.2 and for monthly average concentrations were below 0.1. The results of the comparison between calculated and measured concentrations suggested the use of APRAC3 for air quality analysis in urban transportation planning in Milan.