The present study determined the effect of genetic obesity and phenobarbital (PB) treatment on the expression and regulation of the hepatic cytochrome P450 enzyme (CYP2C11) in Fa/? and fa/fa Zucker rats. Hepatic CYP2C11 levels as determined by Western immunoblotting and associated enzymatic activity (testosterone oxidation at the 2 alpha position) were significantly lower in untreated fa/fa Zucker rats compared with that observed in Fa/? Zucker rats. There was no significant difference in the constitutive CYP2C11 steady-state mRNA level hybridizable to the cDNA (P450 16 alpha) or specific oligonucleotide probe (Northern and slot blot analyses) between fa/fa and Fa/? Zucker rats. The depressed constitutive CYP2C11 protein levels in fa/fa rats may be attributed to their low plasma testosterone and growth hormone levels; however, lack of differences in CYP2C11 steady-state mRNA suggest post-transcriptional regulatory mechanism(s). Treatment with PB further suppressed hepatic CYP2C11 protein levels and activities in both fa/fa and Fa/? Zucker rats in comparison with that seen in controls. The level of CYP2C11 steady-state mRNA was significantly higher after treatment with PB in Fa/? Zucker rats, while no change was observed in fa/fa animals. The mechanism by which PB treatment fails to increase CYP2C11 steady-state mRNA levels in the fa/fa Zucker rat is unknown; however, it may share a common molecular basis with the defect in nuclear transcription rate previously observed with CYP2B1/2B2.