This study summarizes seasonal variations of NOx and O-3 concentration levels. Samples were taken between May 2001 and April 2002 in a monitoring site located in the city center of Bursa, Turkey. Owing to the emission rates and meteorological conditions, air pollutant concentrations varied. The annual average NOx and O3 concentrations were found to be 78.96 +/- 70.39 mu g/m(3) and 55.10 +/- 32.53 mu g/m(3), respectively. In general, the maximum NOx concentrations (115.35 +/- 38.83) occurred during the winter while minimum values (44.86 +/- 17.10) were observed during the summer. This fluctuation probably caused by the residential heating, mixing height and photochemical activity in the region. On the other hand, ozone concentrations were higher in the summer by the effect of photochemical oxidation. This study may help in forensic investigations dealing with O-3 sources in the atmosphere.