Endocrine disrupting chemicals (EDCs) interfere with the function of the hormone system of all vertebrates and several non-vertebrate species. A variety of EDCs are found in aquatic environments from industrial contamination and from wastewater, where urinary estrogens and synthetic estrogens used for birth control are important contributors. Evidence from field studies suggests relationship between environmental EDC exposure and reproductive alterations in fish, and feminisation of male fish living downstream of sewage treatment plants has been observed. Effects of estrogenic chemicals on reproductive variables, such as abnormal gonad structure and differentiation, intersexuality and sex reversal, decreased sperm count and expression of egg yolk protein in males are well established in many fish species. Studies in mammals and to some extent in fish have shown that EDC exposure during development affect not only fertility and reproduction, but also brain development and sexual and nonsexual behaviour. Changes induced in developing organisms seem to be largely irreversible. New experimental data, so far on rodents only, indicate that effects can be transmitted to offspring several generations later by epigenetic mechanisms, raising the possibility that EDCs can affect long-term fitness in wildlife and human. In the present project, we study if trans-generational effects of EDCs on reproduction and sexual and non-sexual behaviour can be identified in fish. We search for new biomarkers identifying inherited epigenetic changes, and will use them to monitor the Baltic marine ecosystem for signs of multigenerational insult by EDCs. Biomarkers detecting persistent effects of developmental exposure in wild populations will be important even if trans-generational effects are not observed. Furthermore, we have found that non-reproductive behaviour, affecting ecologically significant parameters such as anxiety and shoaling, are very sensitive to EDC exposure, and will further study the mechanism of interference with brain signalling. The main questions addressed are: – Can endocrine disrupting chemicals give reproduction disturbing trans-generational effects in fish? – Can biomarkers of persistent changes caused by exposure be identified, possibly connected to transgenerational effects that can be used to indicate reproduction disturbances in natural fish populations? – Are non-sexual behaviour and brain function important targets for EDCs in fish?