The effect of caloric restriction (CR) on the metabolism and DNA binding of 7-chlorobenz[a]anthracene (7-Cl-BA), a tumorigenic environmental contaminant, by liver microsomes of 9-month-old male B6C3F₁ mice and 9- and 12-month-old female B6C3F₁ mice was studied. Mice were fed ad libitum (AL), or starting at 14 weeks of age, received 60% of the calories consumed by control mice and were sacrified at 9 or 12 months of age. After microsomal incubations, metabolites were extracted, purified by reversed-phase HPLC, and identified by comparison of their UV-visible and mass spectral data with those of standards. In each case, ten metabolites were identified: 7-Cl-BA trans-3,4-dihydrodiol, 7-Cl-BA trans-5,6-dihydrodiol, 7-Cl-BA trans-8,9-dihydrodiol, 7-Cl-BA trans-10,11-dihydrodiol, 7-Cl-BA 5,6-epoxide, and 4-, 5-, 6-, 8-, and 9-OH-7-Cl-BA. The quantity of 7-Cl-BA trans-3,4-dihydrodiol was formed in a higher yield from the AL microsomes than from the CR microsomes. Metabolism was also performed in the presence of calf thymus DNA, and the resulting modified DNA was isolated and analyzed by both ³²P-postlabeling/TLC and ³²P-postlabeling/HPLC. Only one DNA adduct, which was derived from 7-Cl-BA trans-3,4-dihydrodiol, was detected in all cases. The levels of adduct formation from CR microsomes were lower than those from the AL microsomes of the same sex and same age. Thus, our results indicate that CR inhibits the formation of the proximate metabolite, 7-Cl-BA trans-3,4-dihydrodiol, and significantly reduces the DNA adduct formation.