The arylhydrocarbon receptor (AhR) is known to mediate toxic responses to dioxin (2,3,7,8-tetrachlorodibenzo-p- dioxin) and related compounds and has been extensively characterized from a toxicological viewpoint. However, it has recently been reported that the AhR may have a central role in ovarian physiology. To investigate the role of AhR during oocyte maturation, we analyzed the expression of AhR, its nuclear partner AhR nuclear translocator, and the major target gene CYP1A1, in bovine cumulus-oocyte complexes (COCs) by semiquantitative RT-PCR and Western blot. Coexpression of AhR and AhR nuclear translocator was observed in both oocytes and surrounding cumulus cells before and after in vitro maturation (IVM). Furthermore, after IVM, both cell types showed a clear up-regulation of AhR mRNA compared with the expression at 0 h. Constitutive expression of CYP1A1 mRNA was observed in immature oocytes at the background level, whereas no expression was observed in the surrounding cumulus cells. Interestingly, a significant increase in CYP1A1 expression level was observed in both oocytes and cumulus cells after IVM. To further investigate the role of AhR in CYP1A1 up-regulation and oocyte maturation, COCs were treated throughout IVM with the AhR antagonists, alpha-naphthoflavone and resveratrol. Both antagonists decreased the level of CYP1A1 in COCs compared with controls. Furthermore, CYP1A1 down-regulation was accompanied by a reduced ability of oocytes to complete in vitro maturation until metaphase II stage. These results suggest that CYP1A1 induction in COCs is necessary for correct proceeding of in vitro oocyte maturation in bovine and suggest a physiological role of AhR during resumption of meiosis.