Recent studies have shown that 17beta-oestradiol (E2) is present in sewage treatment works effluents at concentration ranging from 2.7 up to as high as 220 ng/L. It is believed that exposure to these concentrations could lead to endocrine modulating effects. In this study zebrafish (Danio rerio) were exposed for 21 days to enviromnentally relevant concentrations of estradiol (5, 25 and 100 ng/L) encompassing either their embryo-larvae (from fertilisation to 21dpf), juvenile (from 21dpf to 42dpf) or adult life stages (>200dpf) with a view to investigating the most sensitive life stage of the zebrafish to E2. Actual tank water concentrations of E2 were measured by GC/MS. For all exposure groups, reproductive outputs (egg number, fertilisation success) were determined. Gonad development (histology) and plasma vitellogenin were measured at the end of each of the E2 exposure period. Effects on secondary sexual characteristics (manifestation of the uro-genital papillae in males) was determined in adult fish. In the juvenile fish exposures, no effect of the E2 treatments were seen on sexual differentiation suggesting that the concentrations of E2 were not sufficient to modify the sexual orientation. However, there was a delay in the age of the fish at their first laying when exposed to E2 (100 ng/L) and also a dosedependent reduction in egg production. In the 100ng/L E2 exposure, a higher proportion of males also developed an uro-genital papillae. These reproductive data for the juvenile fish exposures, together with the biomarker results, will be compared to those for the embryo-larvae and adult exposures.