${eta}-lonone$ has been reported to induce the cytochrome P45O (P45O) 2B1 in rats. In this study, the effects of ${eta}-ionone$ and an isomer, ${alpha}-ionone$, on liver P45O IA and 2B expression in Sprague Dawley rats were investigated . Subcutaneous administration of ${alpha}-$ and ${eta}-lonone$ 72 and 48hr prior to sacrificing the animals induced the liver microsomal P45O 1A and 2B proteins. P45O 2Bl induction was associated with the accumulation of its corresponding mRNA. 1 Induction by ${eta}-lonone$ was much higher than that by ${alpha}-ionone$-ionone in both the mRNA and protein levels. When the route of administration was compared, P45O 2B was induced more strongly after oral administration compared to that after subcutaneous injection. A single oral dose of 100, 300 and 600 mg/kg of ${alpha}-$ and ${eta}-lonone$ for 24 h induced P45O 2B1 -selective pentoxyresorufin Odepentylase activity comparably in a dose-dependent manner In addition, ${alpha}-$ and ${eta}-lonone$ induced the P45O 1A and 2B proteins. These results suggest that ${alpha}-$ and ${eta}-lonone$ might be potent P45O 2Bl inducers in rats, and that both ionones may be useful for examining the role of metabolic activation in chemical-induced toxicity where metabolic activation is required.