The project goal was to support the regulatory acceptance of data generated with the ⁠OECD⁠ Acute Fish Embryo Toxicity Test (TG 236) as alternative method to the OECD Acute Fish Toxicity Test (TG 203). TG 236 is an important contribution to animal welfare in chemical safety assessment in Europe but also worldwide. Results of the project are directly used within OECD project 2.54 on the Development of a Guidance Document for an Integrated Approach to Testing and Assessment of Acute Fish Toxicity. Specific results: (1) the knowledge on xenobiotic transformation capacities in zebrafish embryos, juveniles and adults is fragmentary. Therefore, transfer of biotransformation data from juvenile fish to adult fish does not appear justified. (2) The chorion is no barrier for uncharged industrial chemicals of a molecular size of 3,000 – 4,000 Da.