Acute exposure of humans to 0.4 ppm ozone results in reversible respiratory function decrements, and cellular and biochemical changes leading to the production of substances which can mediate inflammation and acute lung injury. The purpose of the study was to determine whether inflammatory changes occur relatively rapidly (within 1h) following exposure to ozone, or if the cascade of events which are initiated by ozone and lead to inflammation, take some time to develop. The authors exposed 10 healthy volunteers twice: once to filtered air and once to 0.4 ppm ozone. Each exposure lasted for 2h at an exercise level of 60 1/min, and bronchoalveolar lavage was performed 1h following exposure. The data from the study were exposed to 03 under identical conditions except that bronchoalveolar lavage was performed 18h following exposure. The results of the present study demonstrate that 03 is capable of inducing rapid cellular and biochemical changes in the lung. These changes were detectable as early as 1h following a 2h exposure of humans to ozone. The profiles of these changes were different at 1h and 18h following ozone exposures. Bronchoalveolar lavage fluid had levels of neutrophils, IL-6, and prostaglandin E2 that were higher at 1h than at 18h post exposure.