Single exposures to low concentrations of ozone (0.4 to 0.5 ppm) have resulted in decrements in forced vital capacity and specific airway conductance. To establish whether adaptation might occur with repeated exposure, 14 normal human subjects were exposed on 5 consecutive days to 0.4 ppm of ozone for 3 hours per day in an environmental chamber. Measurements of forced vital capacity and specific airway conductance obtained after exposure to ozone were compared to corresponding control values obtained during the previous week, when the same subjects breathed filtered air in the environmental chamber for 3 hours per day on 5 consecutive days at the same time of day. The forced vital capacity was significantly lower than the control value on the first 3 days of exposure to ozone, but there was no significant difference on the fourth and fifth days. Specific airway conductance was significantly lower than the control value on the first and second days of exposure to ozone; no significant difference was noted on the third, fourth, or final day. All subjects were symptomatic on the first and second days of exposure to ozone. Symptoms resolved thereafter, with only one subject remaining symptomatic on the final day of exposure to ozone.