Asthmatics performed three 10-min moderate exercises while exposed to 0.00 and 0.30 NO2. Following the initial exercise, FEV1 decreased to a greater extent (P<0.05) in NO2 than in air (7%). After the second and third exercises, decreases in FEV1 and FVC and increases in SRaw were significantly greater in NO2 than in air. These results prompted the conduct of a more comprehensive concentration-response study. Asthmatics were exposed to 0.0, 0.15, 0.30, and 0.60 ppm NO2; again 10-min moderate exercises were included. When compared to air exposure, there were no pulmonary responses after any exercise in any concentration of NO2. Airway responsiveness to methacholine after NO2 exposure was also not different from that after air exposure. The findings of no effects in this larger concentration-response study do not support the findings of preliminary single-concentration study. Possible explanations are differences in subject sampling, variability in subjects' responses, or other factors such as seasonal variation in response. (Copyright (c) Princeton Scientific Publishing Co., Inc.)