The estrogenic activity of 17beta-estradiol (E2), alpha-zearalenol (alpha-ZEA), genistein (GEN), and 4-t-octylphenol (4-t-OP) was investigated using Xenopus laevis-based assays. All test compounds competed with [3H]E2 for binding to a recombinant Xenopus estrogen receptor (xER) with the following relative affinities: E2 > alpha-ZEA > 4-t-OP > GEN. The ability of these compounds to induce xER-mediated reporter gene expression was then assessed in MCF-7 human breast cancer cells cotransfected with a Gal4-xERdef chimeric estrogen receptor and a Gal4-regulated luciferase reporter gene. Luciferase activity was increased 30- to 50-fold by 10 nM E2 relative to that in solvent control. Maximal reporter gene activity induced by 10 nM alpha-ZEA was 54% of that induced by E2; however, the activity did not increase following doses of up to 10 microM. A dose of 1 microM 4-t-OP induced 23% of the maximal reporter gene activity induced by E2, whereas 10 microM GEN induced activity to the same level as E2. A dose-dependent increase in vitellogenin (VTG) mRNA expression was observed in Xenopus treated intraperitoneally with E2 at 0.05 to 5 mg/kg/d for three consecutive days, with the maximal induction observed in the group receiving 1 mg/kg/d. The alpha-ZEA, GEN, and 4-t-OP also significantly induced VTG mRNA expression, although at higher doses. These results demonstrate the utility of X. laevis as an amphibian model to assess the estrogenic activity of endocrine disruptors.