The mammalian constitutively active receptor (CAR) is a novel ligand-activated transcription factor that participates in controlling the expression of cytochrome P450 2B (CYP2B) genes in response to pharmaceutical agents (phenobarbital) and halogenated aromatic hydrocarbons (ortho-substituted PCBs). The occurrence and physiological function of this protein are as yet unknown in marine animals, where there has been a paradoxical lack of induction by PB-type chemicals. One approach to understanding CAR function is to study the evolutionary history of processes such as CAR-CYP2B coupling. In this study, CAR function was evaluated in a representative teleost fish (scup, Stenotomus chrysops). Treatment of scup with 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), which is one of the most potent CYP2B inducers in mouse, caused no increase in hepatic alkoxyresorufin O-dealkylase activity nor in immunodetectable CYP2B-like protein levels. Western blot analyses of scup livers using anti-human CAR antisera revealed the occurrence of a putative CAR homologue in nuclear and cytoplasmic fractions, but no nuclear accumulation of CAR following TCPOBOP treatment, which is a first step regulating the transcriptional activation of CYP2B genes in mouse. Immunohistochemical study also showed no translocation of CAR into nucleus in the hepatocytes of TCPOBOP-treated scup. These results suggest that there may be species-specific differences in CAR activation or CAR-CYP2B coupling signaling transduction in fish from those in mouse. © 2002 Elsevier Science Inc. All rights reserved.