The formation of 2,3-dihydroxybenzoic acid (DHBA) from salicylate was measured in the lungs and plasma to assay for the .OH in juvenile (2 mo), adult (9 mo), and senescent (24 mo) Fischer 344 male rats exposed to clean air, 1 part/million (ppm) ozone, or 2 ppm ozone for 2 h. Similar rates of distribution of salicylic acid in plasma and to the lungs were observed among air control animals of all age groups. Levels of 2,3-DHBA were about twice as high in the lungs of air control senescent rats compared with juvenile and adult rats (P < 0.05). Exposure to ozone resulted in 1.5- to 2-fold elevation of 2,3-DHBA in lungs and plasma of all age groups (P < 0.05), whereas levels of 2,5-DHBA were not changed significantly. There was no effect of age on the magnitude of 2,3-DHBA increase in the lungs or plasma after ozone exposure. The postulated source of .OH is chemical reduction of H2O2, which could be generated from increased age-dependent endogenous oxidant production or the age-independent reaction of ozone or macrophage-derived oxidants with the surfactant lining.