To evaluate the acute effects of NO2on the physical performance of experimental animals, male mice 15–16 wk old were exposed to 5, 10, 20, or 40 ppm NO2for 24 h, and forced swimming endurance time (FSET), changes in FSET during the postexposure days, lung wet weight, lung water content, and blood lactate level were examined. FSET of mice exposed to NO2at 10, 20, and 40 ppm decreased immediately after exposure. FSET of mice exposed to 5 ppm NO2did not change immediately after exposure, but a significant decrease was observed from d 1 to d 4 after exposure. FSET recovered to the initial control level after 5–6 d, 7–8 d, and over 9 d in mice exposed to NO2at 5, 10, and 20 ppm, respectively. Lung wet weight of mice exposed to 20 and 40 ppm NO2and lung water content of mice exposed to 10, 20, and 40 ppm NO2increased with dose. A negative relation was demonstrated between FSET and lung wet weight and between FSET and lung water content. Without forced swimming exercise, the blood lactate level of mice exposed to 5 ppm NO2was almost equivalent to that of control mice, but the blood lactate level of the exposed mice significantly increased immediately and 24 h after forced swimming exercise for 4 min.