2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) are two important heterocyclic amines formed in proteinaceous foods during the cooking process. Both PhIP and IQ are carcinogenic in several strains of rats. PhIP induces mammary tumors in female F344 rats, while IQ induces principally mammary and liver tumors in female Sprague-Dawley rats. Both PhIP and IQ are activated enzymatically, first by N-hydroxylation, catalyzed by CYP1A1 and CYP1A2, and subsequently by esterification (O-acetylation or sulfation), to yield DNA adducts. Such DNA adduct formation, and persistence of adducts, is related to initiation of carcinogenesis, while inhibition of this process leads to prevention of carcinogenesis. Indole-3-carbinol (I3C), a constituent of cruciferous vegetables, has chemopreventive properties in various systems; it probably acts by induction of detoxification enzymes. We have examined the effect of dietary I3C on DNA adduct formation by PhIP in female F344 rats and on that by IQ in female Sprague-Dawley rats. In experiment 1, F344 rats were maintained on AIN-76A diet containing 0.1% (w/w) I3C and then given p.o. doses (10 or 50 mg/kg) of PhIP. These doses are known to induce CYP1A1 and CYP1A2. Groups of animals (4/time point) were euthanized 1, 2, 6, and 16 days later, and their blood (for isolation of white blood cells), mammary glands, liver, stomach, small intestine, cecum, colon, heart, lungs, kidneys, and spleen were removed for DNA isolation and quantitation of PhIP-DNA adducts by 32P-postlabeling. PhIP-DNA adduct formation was inhibited (40-100%) by I3C in virtually all organs, including the mammary gland (the target organ), at both doses of PhIP, and at almost all time points. In a second experiment, Sprague-Dawley rats were fed either control AIN-76A diet or this diet containing 0.02% I3C or 0.1% I3C for a total of 42 days. IQ was added to the diets (0.01%, w/w) from day 15 to day 42, after which all rats received diet free of IQ and I3C. Groups of animals (4/time point) were killed on days 43 and 57. In addition to the organs removed in experiment 1, the pancreas, uterus, and ovaries were also removed, and IQ-DNA adducts were quantitated by 32P-postlabeling. Both dietary concentrations of I3C inhibited IQ-DNA adduct formation in most organs (except in lungs, kidneys, and pancreas) on both days 43 and 57; in liver, stomach, mammary gland, and spleen, inhibition was evident only on day 43. Inhibitions ranged from 22.6 to 86.6% with the 0.02% I3C diet and from 32.2 to 89.6% with the 0.1% I3C diet. I3C diets did not affect rate of adduct removal in either experiment. It is concluded that dietary I3C inhibits PhIP- and IQ-DNA adduct formation in both target and nontarget organs of female rats, even with high doses of PhIP when CYP1A1 and CYP1A2, the enzymes responsible for the initial activation (N-hydroxylation) of PhIP, are expected to be induced.