Curcumin has been shown to possess anti-initiating and anti-promoting activity in experimental systems. However, the mechanisms of its actions are not fully elucidated in vivo. In the present study, mechanisms of curcumin-mediated anti-initiation were investigated in mice employing benzo[a]pyrene (B[a]P) as a model carcinogen. Dietary pretreatment of mice with chemopreventive doses of curcumin showed significant inhibition of B[a]P-induced enzyme activity, protein and messenger RNA (mRNA) levels of cytochrome P450 1A1/1A2 in liver and lungs. Although curcumin alone did not alter the basal levels of aryl hydrocarbon receptor (AhR), it significantly decreased the B[a]P-induced AhR protein levels, its phosphorylation, nuclear translocation and subsequent binding to DNA, thereby decreasing the transactivation of CYP1A. Dietary curcumin led to increase in NF-E2-related factor-2 (Nrf2) protein levels and enhanced its nuclear translocation in liver and lungs of mice as compared with controls. Additionally, increased binding of Nrf2 to antioxidant response element occurred in nuclear extracts from liver and lungs of mice pretreated with dietary curcumin. Induction of activity, protein and mRNA levels of glutathione S-transferase, its isoforms and NAD(P)H:quinone oxidoreductase-1 by dietary curcumin in mice paralleled the curcumin-mediated activation of Nrf2, leading to increased detoxification of B[a]P. In agreement with the observed curcumin-mediated decrease in B[a]P-induced phase I enzyme and concomitant induction of phase II enzymes, pretreatment with dietary curcumin resulted in significant reduction of B[a]P-induced DNA adduct, oxidative damage and inflammation. To conclude, curcumin exhibits anti-initiating effects via modulating the transcriptional regulators of phase I and phase II enzymes in mice.