The authors exposed 22 healthy adult non-smoking men for 2 hours to either filtered air, 1.0 ppm sulfur dioxide, 0.30 ppm ozone or the combination of 1.0 ppm sulfur dioxide plus 0.30 ppm ozone. It was hypothesized that exposure to near threshold concentrations of these pollutants would show any interaction between the two pollutants that might have been masked by the more obvious response to the higher concentrations of ozone used in previous studies. Each subject alternated 30-minute treadmill exercise with 10-minute rest periods for the 2 hours. Following ozone exposure alone, forced expiratory measurements (FVC, FEV-1, and FEF25-75%) were significantly decreased. The combined exposure to SO2 plus 03 produced similar but smaller decreases in these measures. There were small but significant differences between the ozone and the ozone plus sulfur dioxide exposure for FVC, FEV-1,-2,-3, and FEF25-75% at the end of the 2 hour exposure. It was concluded that, with these pollutant concentrations, there is no additive or synergistic effect of the two pollutants on pulmonary function.