In this study, in vitro estrogen receptor (ER) competitive binding and gene expression assays, and the induction of vitellogenin mRNA in vivo were used to assess Xenopus laevis as an amphibian model for examining potential endocrine disruptors. Competitive binding to Xenopus ER was investigated using a bacterially expressed fusion protein consisting of glutathione-S-transferase (GST) linked to the ER ligand binding domain (LBD) of Xenopus (GST-xER). Equilibrium analysis revealed saturation was reached at a concentration of 10 nM [³H]17b -estradiol (E2) with a dissociation constant (K_d) of 6.4 ± 1.3 nM. In a competitive binding assay, the IC₅₀ value for E2 was 12.2 ± 0.34 nM. The ability of E2 to induce Xenopus ER-regulated gene expression was assessed in MCF-7 human breast cancer cells transiently transfected with a chimeric receptor consisting of the Gal4 DNA binding domain linked to the Xenopus ER LBD (Gal4-xER) and a Gal4-regulated luciferase reporter gene, 17m5-G-Luc. Treatment of E2 resulted in approximately 30-50 fold maximal induction of reporter gene activity with an EC₅₀ of 0.67 + 0.31 nM. Adult male Xenopus were intraperitoneally treated for 3 consecutive days with E2 at 0, 0.05, 0.1, 0.5, 1, and 5 mg/kg for a period of 12 days. The level of vitellogenin induction was determined using semiquantitative RT-PCR. E2 treatment showed a dose response increase in vitellogenin mRNA with the maximal induction at an accumulative dose of 3 mg/kg E2 (p < 0.001, n = 35). In summary, we have developed three complementary methods to detect estrogenic effects of E2 using Xenopus as a model. These methods can be used to assess the estrogenicity of a substance in Xenopus as a model amphibian species.