Caloric restriction is known to reduce chemically-induced tumor incidence in laboratory animals. The effect is believed to be mediated in part by modification of hepatic drug metabolism, including both phase I and phase II enzymes. Using aflatoxin B1 (AFB1) as a model carcinogen, we studied the effect of caloric restriction on the modification of rat liver nuclear DNA by AFB1 and DNA damage due to the formation of apurinic sites from the AFB1-DNA adduct removal process. Caloric restriction reduced the metabolic activation of AFB1 which resulted in a decrease of AFB1-DNA binding by more than 50%. The results of the study of the effect of caloric restriction on the AFB1-induced DNA strand breakage assayed by the alkaline unwinding technique showed that caloric restriction protected the formation of apurinic sites from the AFB1-DNA adducts and reduced the double strand DNA breakages by 1.3-2.5-fold. Thus, the lower initial AFB1-DNA binding and less DNA damage, presumably by the less apurinic sites formed during the depurination process of AFB1-DNA adducts, contributed to the protective effect of caloric restriction.