Basal lung concentrations of ascorbic acid (AA), nonprotein sulfhydryls (NPSH), and a-tocopherol (a-T) were determined in rabbits, guinea pigs, rats, hamsters, mice, domestic pigs and sheep, and in human lung samples obtained from cancer surgery patients. Significant differences were found among the laboratory animals: AA levels ranged from 25.6 mg% (mg/100 g wet wt) in hamsters to 41.7 mg% in mice, NPSH from 50.9 mg% in rats to 84.7 mg% in rabbits, and a-T from 1.01 mg% in hamsters to 2.74 mg% in rats. Porcine lung AA and NPSH approximated those of the laboratory animals and their a-T levels were the highest of all species studied. Sheep lung AA levels were comparable to the other species, although their NPSH and a-T levels appeared to be lower. Human lung concentrations of NPSH (5.2 mg%) and a-T (0.85 mg%) appeared low, and the mean AA concentration (22.1 mg%) was similar to those of laboratory animals.