To identify and evaluate the benzidine-DNA adducts in liver and bladder, we exposed the 80 ppm benzidine to 40 sprague-dawley rats by drinking water for 4 weeks(6.2 mg/kg body wt./day). Only one benzidine-DNA adduct was found at the same site of thin layer chromatogram of ^32P-postlabeling method in the liver and bladder of exposed rats. So we know the DNA adduct formed at liver and bladder were similar to each other, which was N-(deoxyguanosin-8-yl)-N`-acetylbenzidine. Relative adduct labeling(RAL) of DNA adduct was similar to each other for 1 and 2 weeks, but that in liver was significantly higher than in bladder for 3 and 4 weeks. RAL × 10 exp(9) of DNA adduct were 84.45 ± 11.31 and 152.8 ± 5.53 in liver, and were 24.76 ± 7.06 and 38.00 ± 10.57 in bladder for 3 and 4 weeks, respectively. Regression equation between liver and bladder was Y=-3.801+2.507 X(r=0.6036, p<0.01). In conclusion, benzidine-DNA adduct formed in liver was significantly higher than that in bladder, with the similar compound structure in sprague-dewley rates treated benzidine in drinking water.