Weanling (52 +/- 4 g) or adult (259 +/- 16 g) male Sprague-Dawley rats were fed ad libitum casein-based diets containing 4 or 16% protein. A third group (food restricted) was fed daily the 16% protein diet, but at the food intake level of the 4% protein group. After 3 wk (weanling) or 5 wk (adults), half of the rats in each group were continuously exposed to 0.64 ppm ozone for 7 d. Ascorbic acid and reduced glutathione levels were then measured. In the heart and liver from weanling rats, ascorbic acid concentrations were lower in the protein-deficient group than in either control group. In the liver from weanling rats glutathione concentrations were also reduced in response to protein deficiency. Exposure to ozone produced no additional response. For adult rats the response for liver glutathione was similar to that of the weanlings. The liver ascorbate concentration, however, was consistently lower in adult rats compared to weanlings exposed to ozone. In lungs from adult rats, the ascorbic acid concentration was lower in the protein-deficient group than in either control group. On a whole-organ basis, both ascorbic acid and glutathione were usually higher in lungs from rats exposed to ozone than from those exposed to air. Interestingly, protein deficiency did not appear to compromise the lung's ability to maintain, in relative terms, the ascorbic acid or glutathione concentration in response to ozone.