Blood from ten young adult male humans, exposed to 1 ppm or 2 ppm nitrogen dioxide (NO2) for 2.5--3.0 hr, was examined for evidence of biochemical changes. The experiments lasted three days. The subjects entered an environmental chamber, performed mild exercise, and completed a series of measurements of pulmonary physiology while breathing filtered air. Blood samples were then taken and analyzed. This regimen was repeated on the second and third day, except that the chamber atmosphere now contained 1 ppm or 2 ppm NO2. Paired group analyses were performed on the data. A statistically significant decrease was observed in the activity of the erythrocyte membrane enzyme acetylcholinesterase at both NO2 levels. Levels of peroxidized red blood cell lipids showed statistically significant elevations after inhalation of 2 ppm NO2 but not 1 ppm. Glucose-6-phosphate dehydrogenase was significantly elevated only after the second 2-ppm NO2 exposure. Small but statistically significant decreases were observed in both hemoglobin and hematocrit values after exposure to both NO2 levels. The experiment was repeated with NO2, (i.e., three days of filtered air) to detect possible effects of the experimental procedure. Decreases were again seen in hemoglobin and hematocrit, and acetyecholinesterase, although of smaller magnitude than when NO2 was inhaled. Other data showed random variations that were not additive over the three-day sham exposure period. It was concluded that significant blood biochemical changes resulted from NO2 inhalation, although the three-day experimental regimen independently produced changes that account for some of the apparent response.