Inductive effects of cytochrome P450 2B1 by $alpha$- and $eta$-ionone were characterized in individual liver lobes of male Sprague Dawley rats. When rats were administered ionones orally at 100, 300, and 600 mg/kg for 24 hr, cytochrome P450 2B1 was induced dose-dependently in liver S-9 fractions as measured by P450 2B-specific monooxygenases and Western immunoblotting. The activity of P450 1A- and P450 2B-specific monooxygenases was differentially expressed in each lobe of normal liver. In addition, the monooxygenase activity was induced by $alpha$- and $eta$-ionone with different potency in each lobe of the liver. Our present results indicate that the different induction of P450s by $alpha$- and $eta$-ionone in each lobe may explain different susceptibilities of rat liver lobes to certain hepatotoxicants which require metabolic activation for their toxicity and that $alpha$- and $eta$-ionone may be useful model inducers of P450 2B1 in studying the toxic mechanism of certain toxicants which may require the metabolic activation by P450.