The potency of ambient air for DNA-adduct formation was estimated using Wistar rats. The animals were maintained in a small-animal facility located beside a main highway intersection in Kawasaki City, Japan, for up to 60 weeks and were exposed to roadside air contaminated mainly with automobile emission (exposure group, EG) or to clean air (control group, CG). Compared to CG, the relative adduct levels (RAL) were increased significantly in EG lungs (17.1-fold (P<0.05)), nasal mucosa, and livers after exposure for 4 weeks. However, there were no significant differences in RAL between EG and CG after exposure for 12 weeks, but they were elevated again in EG after exposure for 48 or 60 weeks. These results suggest that roadside air in this region can cause the generation of DNA adducts. This activity of ambient roadside air can be estimated using experimental animals, indicating that biological monitoring of DNA-adduct formation may be a powerful tool to assess the effect of ambient air on human health.