To determine the capacity of sewage treatment work effluents to disrupt the endocrine system under semifield conditions, two amphibian species, Xenopus laevis and Rana temporaria, were exposed to the effluent of a regional sewage treatment plant in South Bavaria during larval development until completion of metamorphosis. Exposure was carried out in river water (Würm) as a reference, and a 1:12-mixture sewage effluent representing the real situation on the spot, and in a higher concentration of sewage using a 1:2 mixture. An accidental impact of industrial wastewater into the reference and dilution medium, Würm, which was caused by a spate in the respective area during the sensitive period of sex differentiation of amphibian larvae, is assumed to be responsible for the relatively high percentage of females observed by histological analysis in all treatment groups. All of these values were higher than those determined in controls exposed to artificial tap water in laboratory experiments conducted in a comparable study design. Sex ratios between species, revealed by the semifield study with decreasing portions of females from control to 1:12 to 1:2, were strongly correlated. Determination of biomarker-mRNA-levels in Xenopus liver using semiquantitative RT-PCR at the end of the experimental phase, when exposure regime has turned into the initially expected situation with the highest load of potential estrogens in the effluent, followed by 1:2 and 1:12 mixture, resulted in a significant increase of Vitellogenin-mRNA in female juveniles exposed to the highest portion of sewage, whereas expression of both androgen and estrogen receptor-mRNA showed no clear differences. The results concerning the induction of estrogenic biomarkers are in accordance with our findings for estrogen receptor binding of sample extracts from the Würm and sewage taken in parallel at the end of the experiment, when sewage extracts possessed a much higher ability to displace [3H]estradiol from the estrogen receptor than the ones extracted from the mixtures.