Eight-two benzimidazole derivatives have been prepared and tested for the ability to inhibit cytochrome P-450 mediated enzyme activity (aminopyrine N-demethylase) from phenobarbitone-induced rat hepatic microsomes. Using physicochemical parameters and multiple regression analysis, we derived a quantitative structure-activity relationship (QSAR) that describes up to 87% of the data variance in terms of hydrophobic and electronic effects and the molar refractivity of the substituent in the 2-position of the benzimidazole ring.