Estrogen-regulated synthesis of vitellogenin (Vg), a yolk-protein precursor required for reproduction, was monitored to explore the potential antiestrogenic effects of the coplanar polychlorinated biphenyl (PCB), 3,3',4,4',5,5'-hexachlorobiphenyl (3,4,5-HCB), in juvenile rainbow trout (Oncorhynchus mykiss). The effects of 17β-estradiol on 3,4,5-HCB induction of cytochrome P4501A (CYP1A) were also examined. Trout were injected with 3,4,5-HCB (0.25, 2.5, 25, 50, or 100 mg/kg) or a vehicle control, and after 10 weeks, they were sampled or injected with 17β-estradiol (0.1 mg/kg). Markers of vitellogenesis, such as liver somatic index, hepatic estrogen- binding sites, and plasma Vg concentrations, in 17β-estradiol-treated fish were not affected by 3,4,5-HCB. Maximal induction of CYP1A protein and mRNA occurred at doses above 2.5 mg/kg, and 17β-estradiol reduced CYP1A protein content at a single dose (0.25 mg 3,4,5-HCB/kg). Ethoxyresorufin-O-deethylase (EROD) activity was induced by 3,4,5-HCB doses of between 0.25 and 2.5 mg/kg, but induction was reduced at higher doses, indicating that 3,4,5-HCB suppressed CYP1A catalytic activity. In 3,4,5-HCB/17β-estradiol-treated fish, plasma estradiol was significantly reduced at 100 mg 3,4,5-HCB/kg, but the depression was not associated with CYP1A induction or with other antiestrogenic effects. Although CYP1A was induced, 3,4,5-HCB did not interfere with vitellogenesis, which suggests that the PCB congener is not a potent antiestrogen in rainbow trout.