To study the mechanism(s) of turmeric-mediated chemoprevention and to compare the chemopreventive efficacy of turmeric/curcumin(s) against benzo[a]pyrene (B(a)P) and 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK, a tobacco-specific carcinogen), the effects of turmeric/curcumin (C), demethoxycurcumin (dmC), bis-demethoxycurcumin (bdmC) and phenyl and phenethyl-isothiocyanates (PITC and PEITC) on the dealkylation of ethoxyresorufin (ER), methoxyresorufin (MR) and pentoxyresorufin (PR) by rat liver microsomes (in vitro) were studied. These reactions are predominantly mediated by cytochrome P450 (CYP450) isozymes 1A1, 1A2 and 2B1, respectively. In vitro incubation of rat liver microsomes with each of the compounds — C, dmC, bdmC, PITC and PEITC — showed a dose-dependent decrease in carbon monoxide binding to microsomes and also showed a dose-dependent inhibition of CYP 1A1, 1A2 and 2B1 activity, as judged by a decrease in formation of resorufin from respective biochemical probes used. Both the isothiocyanates inhibited activity of CYP 2B1 more readily than that of CYP 1A1/1A2. Significantly lower concentrations of curcumin(s) than isothiocyanates achieved 50% inhibition of activity of CYP 1A1 and 1A2, while concentrations of C (4 μm), bdmC (2.5 μm) required to inhibit CYP 2B1 were slightly higher than that of PEITC (1.3 μm), suggesting curcumin(s) to be effective inhibitors of CYP 2B1 as well. Pretreatment of rats with 1% turmeric through the diet resulted in a significant decrease in induction of B(a)P-induced CYP 1A1 and 1A2 and phenobarbitone (PB)-induced CYP 2B1 in liver, lung and stomach, although the extent of the decrease was different. These results suggest that turmeric/curcumin(s) as in the case of isothiocyanate, PEITC, are likely to inhibit activation of carcinogens metabolized by CYP450 isozymes, namely, CYP 1A1, 1A2 and 2B1.