Swiss Webster Dawls albino mice were exposed to NO2 for 1 hour. At concentrations of 10 ppm of NO2 in females, it was found that hemoglobin concentrations and ery-throcyte counts decreased, and that bilirubin and methemoglobin concentrations increased significantly P ≤ 0.05). At concentrations at 5 ppm, hemoglobin and erythro-eyte counts also decreased, while no changes were observed at 1 ppm. In male mice, at 10, 5, and 1 ppm, there were no changes in hemoglobin and erythrocyte counts. At 10 ppm, bilirubin and methemoglobin concentrations increased. At 10 ppm, there were no changes in either male or female mice in hematocrits, pH, and differential leukocyte counts. It is suggested that the apparent hemolytic anemia is caused by the ions produced by the reaction between NO2 and water in the respiratory system, and that these ions inhibit cellular respiration.