Twenty healthy adults, 10 smokers and 10 nonsmokers, were exposed to 0.5 ppm of ozone for 6 hours in an environmental chamber. They engaged in two 15-min, medium-exercise stints on a bicycle ergometer during this period. The symptoms most commonly noted with exposure to ozone, dry cough and chest discomfort, were experienced by more nonsmokers than smokers. Subjects who experienced symptoms, in general, were those who developed objective evidence of decreased pulmonary function. Significant changes from control values for the group as a whole with exposure to ozone were observed for the following pulmonary function tests: specific airway conductance, pulmonary resistance, forced vital capacity, and 3-sec forced expiratory volume. No significant change was observed with respect to diffusing capacity for CO, static compliance, or the various tests derived from the N2 elimination rate. In addition, nonsmokers exhibited a significant decrease in dynamic compliance after exposure to ozone. When the smokers were considered as a separate group, no significant decrease in pulmonary function was observed, although some individual smokers showed adverse functional changes.