We examined the constitutive and inducible levels of microsomal cytochromes P450 1A1 and 1A2 (CYP1A) in rat cerebral cortex and cerebellum at the level of proteins by western blot analysis, and by catalytic activities via ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD). In the cerebral cortex, cytochrome P450 1A1 (CYP1A1) protein was more abundant than cytochrome P450 1A2 (CYP1A2) protein. Treatment with beta-naphthoflavone (beta-NF) caused a slight decrease in the level of the former but induced the latter 5.8-fold. In the cerebellum, in contrast to the cerebral cortex, CYP1A1 protein was less abundant than CYP1A2 protein in untreated rats, and while beta-NF treatment caused a 3.3-fold induction of CYP1A1 protein, it resulted in a 10-fold decrease in CYP1A2 protein. The CYP1A-preferential activity EROD was 2.3-fold higher in the cerebellum than in the cerebral cortex, and was induced 1.5-fold and 1.9-fold in the cerebellum and cerebral cortex, respectively, by beta-NF treatment. The CYP1A2-preferential activity MROD was 3-fold higher in the cerebellum than in the cerebral cortex, and was repressed 2.2-fold in the cerebellum but induced 3.7-fold in the cerebral cortex following beta-NF treatment. The results show that CYP1A1 and CYP1A2 proteins and catalytic activities are constitutively expressed in brain but are differentially inducible in the rat cerebral cortex and cerebellum.