A series of 2-n-alkylbenzimidazoles inhibited cytochrome P-450 dependent aryl hydrocarbon hydroxylase (AHH) and aminopyrine N-demethylase (APDM) activities in phenobarbitone (PB) induced rat liver microsomes. 2-Un-decylbenzimidazole was the most potent compound in the series, having I50 values of 1.8 X 10(-5) and 1.5 X 10(-5) M against AHH and APDM activities, respectively. Inhibitory activity increased with increasing carbon chain length of the 2-substituent. Regression analysis showed that there was an apparent relationship between inhibitory activity and hydrophobicity (expressed as the octanol/water partition coefficient) for the inhibition of both AHH and APDM activities in PB-induced rat liver microsomes. In contrast, these compounds showed little or no inhibitory activity toward cytochrome P-448 dependent AHH activity in hepatic microsomes from 3-methylcholanthrene (3-MC) treated rats. Two 5,6-dimethylbenzimidazoles showed slight inhibitory activity and naphtho[2,3:4',5']imidazole was only threefold less active toward 3-MC-induced (I50 = 2.6 X 10(-4) M) than PB-induced (I50 = 8.4 X 10(-5) AHH activity. These results suggest that for nitrogen heterocycles there may be a relationship of increasing polycyclic size and increasing inhibitory activity toward AHH activity in 3-MC induced rat liver microsomes.