Two experiments were carried out to determine whether ozone causes significant oxidation of pulmonary polyunsaturated fatty acids in vivo. These involved ad libitum and pair-feeding. In the first experiment, rats were fed fat-free diets and exposed to ozone for 0, 1, 2, and 4 weeks. Lung and liver fatty acids were analyzed to determine if the rats exposed to ozone lost essential fatty acids more rapidly than those exposed to filtered air. In the second experiment, rats were divided into four groups. Two of these groups were fed fat-free diets, and two were fed diets containing essential fatty acids. Rats from the two diet groups (one of each type) were exposed to ozone, while the remaining two groups were exposed to filtered air. In the second experiment, rats were pair-fed. The amounts of lung and liver fatty acids were relatively uninfluenced by breathing ozone. Results from these experiments demonstrate that in the lung, the polyunsaturated fatty acids, linoleic and arachidonic acids, appear to be oxidized by filtered air and ozone at essentially the same rate.