A study of the effects of NO2 inhalation on body weights of newborn and adult mice was carried out. A total of 1590 adult mice and 450 newborn mice were evaluated. The level of NO2 exposure was in the ambient range and varied between 0.17 and 0.80 +/- 0.05 ppm depending on experimental design. The duration of exposure was three to twelve weeks, depending upon specific experimental design. The results have indicated that newborn mice are more sensitive than adults to the inhalation of ambient level NO2 and showed significantly lower body weight gains. The findings were interpreted as being indicative of adverse systemic NO2 effects on newborn animals.