We studied the estrogenic effects of model chemicals in one-year-old juvenile rainbow trout. Methoxychlor (20 mg/kg), diethylstilbestrol (15 mg/kg), 4-tert-octylphenol (25 and 50 mg/kg), and biochanin A (25 and 50 mg/kg) were injected intraperitoneally on days 1, 4, and 7. Fish were sacrificed on day 9 and examined for multiple biomarkers. All of the test chemicals caused increases in plasma vitellogenin levels, a biomarker of estrogenicity. Treatment with the xenoestrogens decreased hepatic lauric acid hydroxylase activity and, as shown by Western blots, also generally reduced expression of hepatic cytochrome P450s 2K1 (CYP2K1), 2M1 (CYP2M1), and 3A27 (CYP3A27) at the protein level. Both doses of biochanin A also significantly induced P4501A (CYPIA) and greatly increased hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity. These findings suggest that methoxychlor, diethylstilbestrol, 4-tert-octylphenol, and biochanin A were all estrogenic and mimicked 17beta-estradiol (E2) in repressing the expression of cytochrome P450 isoforms (CYP2KI, CYP2M1, and CYP3A27) in the rainbow trout liver. Additionally, biochanin A was found to induce CYPIA in this fish species.