Surface water receives a variety of micro-pollutants that could alter aquatic organisms' reproduction and development. It is known that a few nanograms per litre of these compounds can induce endocrine-disrupting effects in aquatic species. Many compounds are released daily in wastewater, and identifying the compounds responsible for inducing such disruption is difficult. Methods using biological analysis are therefore an alternative to chemical analysis, as the endocrine disruption potential of the stream as a whole is considered. To detect hormonal disruption of thyroid and oestrogenic functions, fluorescent Xenopus laevis tadpoles and medaka (Oryzias latipes) fish larvae bearing genetic constructs integrating hormonal responsive elements were used for physiological screens for potential endocrine disruption in streams from an urban wastewater treatment plant. The Xenopus model was used to assess thyroid disruption and the medaka model oestrogenic disruption in wastewater samples. Assays using the genetically modified organisms were conducted on 9 influent and 32 effluent samples. The thyroidal effect of wastewater was either reduced or removed by the treatment plant; no oestrogenic effect was detected in any of the wastewater samples.