Previous research on young adults has indicated that the magnitude of pulmonary function decrements induced by exposure to ambient ozone (O 3) is related to the effective dose of O 3 inhaled. The effective dose is defined as the product of O 3 concentration (in ppm), mean minute ventilation (V E) and duration of exposure (min). The relative contributions of the three components of effective dose to the development of pulmonary function decrements in older adults are unknown. Twelve healthy, nonsmoking men and women (60–79 years) participated in each of four experiments: (1) a 1-h continuous excercise protocol, and (2) a 2-h intermittent exercise protocol, each performed while exposed to filtered air (FA), and to 0.45 ppm O 3, resulting in different effective doses of O 3. Pulmonary function (forced vital capacity, FVC, functional residual capacity, FRC, and associated calculated parameters) was measured pre- and postexposure. Ozone exposure induced significant decrements in forced expiratory volume in 0.5, 1.0 and 3.0 seconds (FEV 0.5, 1.0, 3.0), regardless of the exercise protocol. There were no changes in FVC with any exposure protocol. There were significant decrements in forced expiratory flow rate at 25% and 50% of FVC (FEF25%, FEF50%) and in forced expiratory flow rate between 25% and 75% of FVC (FEF25–75%) with all four exposures, suggesting a fatigue effect. There were no differences between the decrements induced in FEV 1.0 by O 3 exposure under the two exercise protocols. The mean exercise V E was 25.3 1/min for the continuous exercise protocol, and was 25.2 l/min for the three exercise periods of the intermittent exercise protocol. There were no physiologically significant metabolic alterations with any of the four exposures. The results suggest that O 3 concentration is the most important factor within the effective dose concept, followed by exercise V E, followed by exposure duration, the same relative rankings observed in young adults.