Hydrogen peroxide (H2O2) is thought to be a major intermediate in the toxicity of ozone. In a previous study we demonstrated that ATP depletion may play an important role in the H2O2-induced inhibition of the phagocytic functions of alveolar macrophages. Ozone exposure can adversely affect the phagocytic capacities of alveolar macrophages. In the present study we investigated whether a decrease in the cellular ATP concentration may underly the effects of ozone on alveolar macrophages. Neither following single (6 and 12 h) exposure nor repeated (12 h/day for 3 and 7 days) exposures of mice or rats to 0.4 ppm ozone, were decreased levels of ATP found in the alveolar macrophages. In contrast, repeated exposures of mice for 7 days to ozone led to a significant increase (1.5-fold) in the ATP content of the alveolar macrophages. In vitro ozone exposure of rat and mouse alveolar macrophages also did not lead to a decrease in the cellular ATP concentration. These results showed that ATP depletion does not play a role in the toxicity mechanism of ozone for alveolar macrophages.