objective. to evaluate the differences between indoor and outdoor ozone (o3) and nitrogen dioxide (no2) levels at a school located in southwest mexico city. material and methods. indoor and outdoor o3 and no2 levels were measured daily between january and april 1990 by manual monitoring. results. indoor and outdoor concentrations of nitrogen dioxide did not surpass the mexican standard (0.21 ppm). the correlation coefficient between maximum no2 concentrations measured by the city？s local monitoring station and those measured outside the classroom was 0.82 (p< 0.001). regarding ozone, its maximum outdoor concentration was 0.29 ppm and indoor concentrations were on average below 0.06 ppm (maximum = 0.17 ppm). the indoor/outdoor correlation coefficient was 0.72, and for every 1.7 ppm outside, there was 1.0 ppm inside (p< 0.05) conclusions. since the highest outdoors o3 concentrations were observed between 11:00 and 14:00 hrs, it is recommendable to have recess before this time.