This Test Guideline describes a Rapid Estrogen ACTivity In Vivo (REACTIV) Assay in an aquatic assay that utilises transgenic Oryzias latipes (Japanese medaka) eleutheroembryos at day post hatch zero, in a multi-well plate format to identify chemicals active on the estrogen axis. The REACTIV assay was designed as a screening assay to provide a medium throughput and short-term assay to measure the response of eleutheroembryos to chemicals potentially active on the estrogen axis.