A series of arylalkyl and alkyl isothiocyanates, and their glutathione, cysteine, and N-acetylcysteine conjugates were used to study their inhibitory activity toward the dealkylation of ethoxyresorufin (EROD), pentoxyresorufin (PROD), and methoxyresorufin (MROD) in liver microsomes obtained from the 3-methylcholanthrene or phenobarbital-treated rats. These reactions are predominantly mediated by cytochrome P450 (P450) isozymes 1A1 and 1A2, 2B1 and 1A2, respectively. All isothiocyanates inhibited PROD more readily than EROD. Increases in the alkyl chain length of arylalkyl isothiocyanates to C6 resulted in an increased inhibitory potency in these assays; at longer alkyl chain lengths (C8-C10) the inhibitory potency declined. The IC50s for phenethyl isothiocyanate (PEITC) were 47, 46 and 1.8 μM for EROD, MROD and PROD, respectively. Substitution of an additional phenyl group on PEITC also increased the inhibitory potency; the IC50s for 1, 2-diphenylethyl isothiocyanate (1, 2-DPEITC) and 2, 2-diphenylethyl isothiocyanate (2,2-DPEITC) were 0.9 and 0.26 μM for EROD, and 0.045 and 0.13 μM for PROD, respectively. The relative inhibitory potency of PEITC and its conjugates was N-acetylcysteine-PEITC (PEITC-NAC) < glutathione-PEITC (PEITC-GSH) < cysteine-PEITC (PEITC-CYS) < PEITC. The observations that the parent isothiocyanates were more potent inhibitors than the conjugates suggest that dissociation of the conjugate is required for activity. Naturally occurring alkyl isothiocyanates, sulforaphane (SFO) and allyl isothiocyanate (AITC), were very weak inhibitors in the assays. These results suggest the potential of isothiocyanates as structural probes for studying P450 isozymes. In addition, the inhibitory activity of isothiocyanates for PROD correlated with the previously demonstrated tumor inhibitory potency in (4-methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induced A/J mouse lung tumor bioassays, which supports earlier findings that P450 2B1 is one of the major isozymes involved in NNK activation and that inhibition of this isozyme is an important mechanism for the chemopreventive activity of isothiocyanates.