Indole-3-ylcarbinol (13C) is formed during processing of cruciferous vegetables and is suggested to be one of the modulators of drug-metabolising enzymes. Indole-3-ylcarbinol is a far less efficient inducer of hepatic enzymes after parenteral than after oral administration, due to formation of active metabolites in the gastrointestinal tract. As indole-3-ylcarbinol is unstable in weakly acidic aqueous solutions, non-active condensation products may be formed from indole-3-ylcarbinol, that cannot be transformed to the active products when reaching the stomach. The purpose of the present study was to test the ability of the condensation products formed at a pH corresponding to that of fresh vegetable juice to modulate the metabolism of xenobiotics. Indole-3-ylcarbinol was incubated in vitro at room temperature in the dark at pH 5.5 and samples taken at various times, for oral administration to rats and for chemical analysis. Indole-3-ylcarbinol was rapidly transformed into various oligomeric products. The 7-ethoxyresorufin O-deethylase activities (marker of cytochrome Cytochrome P450 1A enzymes, CYP1A) in liver, kidney and colon increased with the duration of the in vitro condensation period whereas the formation of 6beta-, 15beta- and and 2alpha-hydroxytestosterone was not affected significantly, indicating no effect on CYP2C11 or CYP3A enzymes. The hepatic metabolism of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). was increased by indole-3-ylcarbinol condensation products and the 4'-OH-PhIP/N-OH-PhIP ratio was decreased due to a significantly increased formation of the proximate genotoxic metabolite. N-OH-PhIP. The activities of DT-diaphorase and glutathione S-transferase were not changed significantly in the rat organs. These experiments clearly indicate that indole-3-ylcarbinol is not the definitive CYP1A inducer and that indole-3-ylcarbinol at near-neutral pH, is transformed to compounds that are inducers by themselves or may be further converted into inducing compounds in the rat stomach. Also, the enzyme inducing potency of indole-3-ylcarbinol containing vegetable juice is apparently enhanced by incubation in vitro before the intake.