Endocrine disruption is a major concern for the health of wildlife populations. Although many studies have shown reproductive impairment as a consequence of endocrine disruption in adult fish, knowledge of the consequences of endocrine disruption for vertebrate embryonic and larval development is scarce. The zebrafish embryo is an ideal model system to investigate the fundamental mechanisms of endocrine disruption. We will first describe the timing of the normal embryonic activation of the hormone synthesis machinery, as well as the hormone profiles, during the early stages of vertebrate development, which has never been done so far. Secondly, using well-described endocrine disrupting compounds, we will specifically disrupt these profiles and propose mechanisms linking the changed profiles to observed developmental effects. We will then validate the proposed mechanisms of toxicity using targeted disruption techniques, including knockout and morpholino knockdown. This project will offer the information that is needed for follow-up projects to develop assays to specifically assess the risk associated with exposure to different classes of EDCs on embryonic and larval development, allowing discrimination among ER and AR agonism and antagonism, as well as aromatase inhibition. Such assays would fit perfectly within both EU and USA regulatory priorities for screening and prioritizing potential EDCs.