Studies of the human health effects of exposure to a combination of ozone and sulfur dioxide have produced somewhat conflicting results; the possibility of a synergistic enhancement of toxicity when the two gases are present simultaneously remains equivocal. We evaluated the effects of 0.40 ppm sulfur dioxide, 0.40 ppm ozone, and the combination of these two under one environmental condition (25 degrees C and 45% relative humidity). Subjects alternately walked and rested during a 2-hr exposure. Subjects exposed to filtered air or to 0.40 ppm sulfur dioxide showed no significant changes in pulmonary function. When exposed to either ozone or ozone plus sulfur dioxide, significant decreases in maximum expiratory flow, forced vital capacity, and inspiratory capacity were observed. There were no significant differences in response between ozone alone and ozone plus sulfur dioxide exposures, thus, in our subjects on synergistic effects were discernible.