Because large intersubject differences in the magnitudes of response to a single ozone (03) exposure have been observed, the authors undertook to determine if this variability were due to differences in intrinsic responsiveness to 03 or to other factors. Each of thirty-two subjects was exposed to one of five 03 concentrations (0.12, 0.18, 0.24, 0.30, or 0.40 ppm), and each underwent one or more repeat exposures. Separated by from 3 weeks to 14 months. Magnitudes of change for pulmonary function and respiratory timing variables and for reported symptoms were compared for the repeated exposures. Changes induced in forced expiratory spirometric measurements were highly reproducible for up to 10 months and for all tested 03 concentrations 0.18 ppm.